Note: The following is an abridged compilation primarily of Twan's post for MA Arco, with some strategy from GameExtremist's MA Arco thread as well as other responses to those threads in the  Shrapnel game forum Strategy Index
The Old Kingdom is Arcoscephale in the Middle Age. Although its glory days are long past, the ancient Astrologers who aided past kings in building Arcosephale into a mighty empire have recently emerged from their centuries-old seclusion, to restore the old kingdom to world domination once more. Preistesses with great knowledge of healing are trained in ancient temples built during the peak of the old kingdom. The Priestesses are able to scry upon enemy troops and provinces that are located within the god's dominion. The war machine of ancient Arcosephale has not changed much over the past centuries. Cumbersome plate hauberks and long spears are still used, and the cavalry is primitive. Elephants and chariots, unpredictable but devastating, are still used.
MA Arco's major magical strength lies in Astral, with minor paths in Fire,Water,Earth and Nature. Easy access to Communions means powerful battle magic in all paths. MA Arco's heavy foot soldiers include spear-wielding Hoplites and sacred Heart Companions.
- Gold x, Resources y
Slingers... militia level slingers with mapmove 1 and low morale, you may eventually build some in cases of extreme emergency, if you have low ressources and need a good number of patrollers ; can be usefull in early game if you can't find indie archers to take barbarians provinces
Peltast... shielded light infantry with javelins. As all other infantries have spears only, the javelins make the peltast the most offensive type. Their defence is rather good (13 with the shield) ; it's the best light infantry to recruit when you are low in ressources. They may be usefull both in very early game (when you are limited in both ressources and gold, and want some extra fire power) and in mid-late game (when you need to build a lot of meatshields and protection doesn't matter anymore as your ennemies use AP/AN dammages or thugs).
Cardaces... Light infantry with a little better protection but costing 2 more ressources and without javelins. Body protection 7 rarely allows them to survive longer than peltasts (protection 6) even if they have a far better helmet (head 14). The two units having a shield and same defence, I prefer the ones with javelins.
Hoplite... The main heavy soldier. 17+ protection, above average morale (12), a length 5 spear for repells. Their high encumbrance (9), and weak weapon (like for all Arco infantries only 3 dmg) make them unpopular for some players, but in fact they are just the kind of soldiers Arcocephale needs. They are not supposed to kill the ennemies themselves or fight long battles, they are here to protect the mages who are the main dammage dealers of the nation and should destroy anything in less than 10 rounds (or some communions slaves risk to be lost anyway). They cost 30 ressources but only 11 gold, so you'll never find reasons not to use all your forts ressources. Even if you don't plan to use them fast, hoplites always worth to be built. Many nations only heavy troops have higher upkeep cost, and are not really better considering magic rather than weapon damage decides the outcome of most battles.
Hypaspist... These "light" hoplites have an insane morale for a non sacred human troop (14) you can build in any fort. They also have mapmove 2, a lower encumbrance and a good defence/protection combo (13/15+). But they cost far more gold (15) than protection 17 hoplites and so aren't really cost effective as main meatshields. They are better used as morale boosters : having some hyspaspists in each squad make your army less likely to rout. And it's particularly a good idea when you use elephants. In early game a squad of 3 or 4 elephants + 5 or 6 hyspapists is often sufficient to take indies who would repel a force of 5 or 6 elephants only. Later in the game, when elephants become obsolete, hyspapists are still usefull due to their mobility. They often are the best soldiers you can send to reinforce your armies some provinces away from your forts (the others arriving after the battle). Also, for a nation with no cavalry, they can be useful as flankers. If they are not very fast, their high morale will often allow them march to the real enemy rear instead of cancelling their "attack:rear" order when they pass the enemy first squads.
Heart Companion... They are just sacred hoplites with a better morale. Not a bad unit with a bless, but as they are capitol only and have mapmove 1 they don't really justify using a strong one (especially as their encumbrance make earth a mandatory part of any good bless for them, so you can't use the overpowered combos like fire/water). Like hyspapists they are better used as morale boosters, with a benediction in addition to a sermon they reach 18 morale, so 3 heart companions in a squad may be sufficient to protect the same number of elephants from routing (also as heart companions are very slow and well protected, the chances to have some killed, and elephants failing their morale checks as a result are very low). Heart companions are also a good counter if you have to fight an awe/fear based SC in the early game.
Chariot... Size 4 tramplers with good defense and a far better MR than elephants, but human hp, so they are vulnerable against ranged/AoE damage. They have mapmove 3 / ap 18 like the elephants, so if you want to send a squad of elephants 3 provinces away (you have a mounted commander for that) and have it ready to fight at the destination (with a little better morale than 9, and a sufficient number of units to avoid to check morale for each hit), chariots are the unit to add. They can also find uses alone in early game, if you lack money for elephants (often the case if you use a luck/turmoil build) or plan to attack a province defended by good melee units only (their defense make them better than elephants against troops able to deal high amounts of damage like barbarians). In mid/late game, against enemies using MR spells to neutralize your elephants, they are also the only tramplers you may want to build (except if you have too few forts and an urgent need of offensive units, money is better used to recruit more mages).
Elephants... They are tough and strong size 6 tramplers, other than this they are rather crappy (low morale and defence, very low MR, just average protection, high upkeep). You want a lot of them in early game ; later they are usually better dead than costing upkeep and trampling your own troops (which will happen in any consequent battle against a good player once some magic is researched, elephants can easily be terrorized, enraged or enslaved). But keeping some elephants may prove useful even after midgame : they force enemy mages to adapt their script to their possible use (even if your elephants finally "forget" to follow your army for the big battle, the enemy will have scripted spells like rage, panic, enslave or terror, probably not the best use of their mages against your other troops) , and on the battlefield their size 6 allow you to chose which squad will be targeted by enemies using fire:large monster (one or two elephants in the middle of your biggest hoplite squad can save the lives of the twenty chariots doing the real trampling, or avoid you to see a valuable thug killed by a lucky ethereal crossbow shot).
Indie Archers... Arco has no national archer, but having only low damage troops requires recruiting a lot of them, especially in midgame, hoplite + archers offer an excellent synergy. A front rank (consisting of 2/3 heavy hoplites and 1/3 hyspapist) and 3 or 4 times as many indie archers behind is the standard Arco army once elephants start to become obsolete. Build a fort on a province with longbowmen or other good archers if you find one. One mystic/2 has a fire pick + astral and so may be able to cast flaming arrows after a communion, it would be sad to have nobody to exploit this very powerful spell.
Scout... A standard scout with no leadership.
Mounted Commander... Not very interesting, but may have an use even in late game if you play on a big map with clear terrains : it's your only commander with mapmove 3, and Arco don't have air magic for flying boots. He may be the only way to exploit the mobility of elephants, chariots or summoned troops (with a crown of command to lead magical beings).
Hoplite/Hyspapist Commander... In theory may be used to optimize your battle scripts (as leaders with bodyguards, they allow their squads to keep cohesion as they have the speed of their kind of troop). In practice they are rather useless as it's better to use hoplites and hyspapist in mixed squads for a better morale, and to use indie commanders when your forts produce mages.
Strategos... If you have to build a national non mage commander, it's probably the best choice. Their standard bonus is good to have near elephants, or your main meatshield squad. To really exploit the bonus if your army advance a script like "attack one turn, hold, attack one turn, hold, attack one turn, stay behind troops" should be used (with the strategos starting about two squares behind your troop) instead of the classical "hold"x5, "stay behind troops".
Priestess... One of the strengths of Arco, the cheap priestesses are the best recruit-able healers of the dominion world and in addition to their holy powers give the nation a little access to nature magic (N1, also allowing to make them very cost effective researchers with magic 3). But don't overestimate them. Troops healing is generally useful in early game to cure the often crippled elephants, then the interest is very situational : if you happen to have an afflicted pretender or living thug, or some mute or feebleminded mages, healing is invaluable ; out of that, only heal your army when you have nothing else to do (searching nature and holy sites, or even preaching, are generally better uses of priestesses than healing your meatshields). And if their nature level may find some uses on the battlefield (especially if you can give them thistle maces), mystics remain far more useful as non-capitol mages, as they can join communions without a matrix.
Mystic... The mages Arco can build everywhere have the strength and weakness to be very diversified, both in their skills and their global power. Their picks (1S + 100% SFWE + 50% F + 50% W + 50% E) mean for the same price you can get a 1/1 or a 2/1/1/1 mage, and you will often have mage with levels 1 only in 3 or 4 schools. Without astral it would mean rather weak mages, but as all have it, mystics are terrific communicants, allowing you to use about any battle spell of their elemental schools in important battles. Also, if their levels are low, three of their paths are easy to boost. One mystic / 8 has earth 2 and so can make earth boots (and then dwarven hammer). One /4 has water 1 for bracelets (1/8 water 2 to forge a robe of the sea with one) etc... As rings of wizardry can be forged by astrologers (with some S boosters) it means a mystic can reach without empowerment level of 5 in water or 4 in earth. The only path they may have difficulties to develop is fire, as even with a ring of wizardry no booster can be made. It may be a good idea to take fire (or better fire/death for the cheap skulls of fire) on your pretender. Once you have a fire booster a mystic can forge a fire/water staff of elemental mastery*, adding one more to all potential levels.
Astrologer... .Capitol only mages are less diversified, but far more powerful in astral magic. They always have S3, the most interesting having level 4 (1/4) or even 5 (1/160). If you have the money, recruiting astrologers is always the priority for the capitol, you want to have as many S4 as possible as most astral spells are MR negate and extra levels give penetration bonus (and if you are lucky and end with one natural S5, keep him preciously, he'll just need one empowerment to cast wishes), The other paths astrologers can have (F,W,E) are less important, as mystics already have them (and far more chances to be level 2 in an element). Astrologers with fire are powerful in evocation even without a communion (once you have researched astral fires) so are a good choice to support your secondary armies (or for small teams of teleported mages). Those with earth once equiped with boots can summon earthpower so you have teleportable mages able to use the many e3 buffs (or gifts from heaven). The ones with water are the less interesting (but in endgame allow to summon abominations). As a secondary power, astrologers have the fortune teller ability (5% to cancel a bad event on their province), but the way this ability work don't make it sufficiently reliable to base a strategy on this (like taking misfortune without order to fund a powerful pretender -always a bad choice) but it's a good idea to send some astrologers in an important provinces with high turmoil/misfortune (or with a luck scale but enemy dominion).
Sirrush... Only national summon of Arcocephale, they are tough and sacred, have an excellent MR, and are one of the very rare creatures you can summon with astral pearls. They may worth to be used if your astral income is high, but I won't say that their existence is sufficient to justify a bless strategy (or to rush conjuration 5, not the best school for Arco).
- Gems z
With its elephants Arcocephale is a very strong nation in the early game, so having a combat pretender in the beginning isn't as important as with most others. But since your sacred units don't really justify a strong bless you can still afford one, or take excellent scales with a sleeping or imprisoned pretender if you prefer.
In mid-late game, Arco's strength lies in powerful astral mages and communion masters able to use boosted battle magic in many paths. The main weakness of the nation is the lack of good offensive troops once elephants become obsolete, and no easy access to powerful thugs/SC.
This makes Arco one of the most mage-dependent nations : to win big battles after early game Arco absolutely need to engage huge numbers of mages, and to use more gems, boosters and penetration items on the field than others, with some powerful battle spells researched since mid game. It means to shine Arco needs four things : a good number of forts built early to recruit a sufficient number of mystics, a positive magic scale (to compensate the number of mages who will be with armies instead of researching, and to make astral spells harder to resist), a good research plan and a strong gem income.
For the scales, the only requirement are for me a positive magic scale and no negative production. Sloth is a bad choice, as your best troops cost 24 or 30 resources, but as initial expansion is done with elephants, gold and not resources is the limiting factor in early game, so high positive production is not really needed (especially as gateways will allow you to build troops each turn far from the front and teleport them where you want). Mages are not old, so growth isn't a must have, priestesses able to heal troops may even be seen as a good reason to fund your other scales with a death dominion, increased affliction chances handicap others more than you (but on big maps some growth is always good to take for long term income). Order (+Misfortune) or Luck (+Turmoil) look as good. Luck is tempting for the extra gems it provides, and as mages are rather cheap an huge regular income isn't an absolute need, but Order boosts elephant based initial expansion, and later you want a lot of gold to build labs everywhere. The only interest of a very high dominion strength (considering sacred are resource intensive and you have correct preachers) would be to give awe to an awake SC, and one isn't needed, so all my builds use medium dominion strength.
Like said above Arcocephale isn't as pretender dependent as most other nations. As you don't need a combat pretender or a bless (but may use them) there is a very large choice of viable builds, from magic-less monsters to rainbows or very-specialized mages. The national pretenders offer a large choice of titans chassis if you want a SC (including many with air or death, if you want to diversify), and you also have access to the classic undead or immortal pretenders, most immobile types, etc...
Magical Economy Pretenders
Your national mages have no way to produce clams or other gem-producing items, and as a magic-dependent nation Arco needs a huge gem income to shine. Magical economy pretenders are IMO the best option for Arco (if you don't play on a small map or with special economy nerfs) if not the most exciting.
Mother Of Clams
Awake Mother of Rivers, Water 3, Earth 3, Nature 2, Dominion 5, Order 3, Misfortune 2, Magic 1
A Mother of Rivers with nature magic is the best choice to develop a clam based economy since early game as Arco starts with no mage able to make them and no water income. Giving her earth 3 allow to forge a hammer as soon you have the gems, so you just need construction 2 to start to make clams at reduced price ; and it also makes the Mother far better as a super-combatant (able to cast invulnerability she can be equipped with a robe of shadows instead of an armor). At conjuration 5 the Mother of Clams can summon naiads and stop to make clams herself (then the geared mother will be rather used on the field).
Another advantage of this build is the capacity to forge thistle maces for the priestesses at construction 4, and the possibility to use the mother as a thug if someone try to rush you (but I won't say she can be used "as a super-combatant", as she don't have awe trying to take provinces with her alone is a bad idea in early game).
As clams will provide all the gems you need, I've chosen order/turmoil for the income scales, but if you want to really swim in gems, this build can be used with Luck 3/Turmoil 2 instead (or Luck 3/Turmoil 3/Production 1). As "falling ice crystals" (water gems) look like the most common gem event it can boost even more clam hoarding.
A mother of rivers can also be interesting without earth (you'll just have to wait construction 4 and to have an E2 mystic able to forge earth boots then hammer), and you can take magic 3 and a little bless (w4) with the points.
Rainbow Forge Lord
Dormant Forge Lord, Fire 3, Water 2, Earth 3, Death 3, Nature 2, Dominion 5, Turmoil 3, Death 1, Luck 3, Magic 3
This build is based on another kind of magical economy : being able to forge most non astral items at extremely reduced costs, when high luck provide extra gem income (and hopefully money).
The paths allow the forge lord to make all the boosters your mystics may need (including f/w staves of elemental mastery once other boosters are forged), as well as clams or fetishes, most useful SC items etc... all at half the cost with a hammer. The death level also allow the lord to summon spectres in midgame (or bane lord once a skull staff is forged), and skullface can be made at -25% cost, making easier to reach the tartarian level. Of course, with such a pretender, it's a good idea to race for construction 8 rather than developing endgame battle magic first.
Offering more possibilities in late game, this build is weaker in early game than the Mother of River : -23% money can be a huge handicap if you don't get some income events fast and you don't have a pretender to use on the field in case of early emergency (but once cured by the priestesses, the forge lord is a slightly better combattant). Dominion is also low for a non-awake god, but it's rather an advantage in early game to have some provinces without your bad income scales.
Astral Rainbow Girl
Awake Enchantress, Astral 6, Earth 2, Fire 1, Water 1, Death 1, Nature 2, Dominion 5, Heat 1, Death 1, Magic 3 (Luck 3/ Turmoil 3 or Order 3/ Misfortune 3 can be added)
Here the idea is to have both an increased astral income and someone able to cast level 9 astral spells without empowerment, when magic 3 + an awake researcher allow you to reach endgame spells before anyone else. Out of astral, paths allow to forge useful boosters like skull of fire, thistle mace then moonvine bracelet etc. (as well as fetishes, and some clams once 2W boosters are forged), but the enchantress will mostly be used as a researcher in the beginning (with construction 6 as a priority, so lightless lantern make the small early research advantage snowball into a big one). Then alteration 9 (wish) or enchantment 9 (arcane nexus) are the logical priorities.
Dormant Wyrm, no magic, Dominion 6, Luck 3 or Order 3 (but no Turmoil/Misfortune), Productivity 2, Growth 2, Magic 3 (or dormant Dragon with 1 less dominion and just his base magic ; dormant Manticore if you like to use the "call god" order a lot)
A big magic-less monster without awe looks strange, but with astral and water mages to buff him (body ethereal, luck, quickness) he can be very dangerous once some spells are researched (so better to take him dormant for great scales). Build more priestesses than usual with this kind of build (with magic 3 they can be the most part of your researchers) so if he is killed you can call back your pretender in one turn (and he has no magic skill to lose and can be cured if needed). So, you have excellent scales and a quasi-immortal beast as a bonus.
Cyclops of the Shroud
Dormant Cyclops, earth 9; Dominion 6, Order 3*, Misfortune 3, Magic 3 (or Awake Cyclops, earth 9, Dominion 6, Order 3*, Heat 1, Death 1, Misfortune 3, Magic 1; or Imprisoned Cyclops, earth 9, air 2, Dominion 6, Order 3*, Growth 1, Misfortune 3, Magic 3) (* or same pretender with Luck/Turmoil scales, or less misfortune and more temperature)
A high level earth god able to create a lot of living statues per cast compensates a weakness of Arcocephale : the lack of good summons (of course you need to adapt your research plan, to develop enchantment 6 in midgame). In addition it provides a very useful bless to both heart companions and... mages. What..? The mages are not holy ? With a shroud of the battle saint they are ! And Arco has no difficulty to forge a lot of them, giving all equipped mages protection 10 and re-invigoration 4 (if you add the boots of the messenger your priestesses can forge, it means you can use terrible classic communions, with all mages staying at 0 fatigue forever ; or half-reverse with all mages casting 10 rounds or more). Protection 10 is also often sufficient to have astrologers surviving the first hit, so it offers a good synergy with the use of ritual of returning. In battle the cyclops can be used as SC, or a terribly efficient caster of Curse of Stones (+3 penetration not counting items and boosters) or Blade Winds (if you use living statues or other troops with high natural protection as meatshields, Iron Bane + 4 x Blade Wind will humiliate any normal army).
An option is to take the cyclops awake and use him as early game SC (as you priestess can cure him when he becomes blind, the main weakness of the chassis is not a problem). Another is to take him imprisoned to give him some air magic, so he is far more mobile (cloud trapeze), and in battle can use rain of stones, an excellent spell considering you have access to an infantry (living statues or blessed heart companion) with 20+ protection, and mages able to cast body ethereal (75% chance to avoid mundane damage) ; and with a matrix to lead a communion he can also use fog warriors.
Mother of Lamias
Dormant Mother of Monsters, nature 8, astral 1, death 2, Dominion 6, Order 3, Misfortune 2, Heat 1, Magic 1 (or very bad scales and Nature 9 or 10, but I don't think such a high nature level worth its cost when you have no recruit-able sacred giants)
Lamias are another option to give a cost effective troop summons to Arcocephale (here you need to develop conjuration 6 asap). Without body or feet slots, and taking extra fatigue in cold provinces, the mother of monsters is a one trick pony : extremely vulnerable as a super-combatant she should always stay in a fort summoning troops (or forging), but her bonus of +3 lamias per cast, coupled with high level nature, is not negligible (with a ring of sorcery + the treelord staff and moonvine bracelet she can forge, she will summon 15 lamias each turn for only 4 nature gems). In addition the nature 8 bless, not bad for heart companions, is clearly excellent for sirrushes, and as conjuration is a logical priority there is no reason not to use them.
In the "specialized summoners" category, the ghost king is also available, but offers no particular synergy with the nation (mages don't have undead leadership, very high dominion has no particular interest, you can cast body ethereal if you need ethereal beings, etc).
Master of Death-Thugs
Imprisoned Master Lich, 5S 4D 2E 2A 2N, 3 order, 1 sloth, 1 cold, 3 growth, 1 magic.
This ties in to the guide's astute observation that when elephants become obsolete, you need thugs to contend. Rather than tie into death, I embraced it as a major theme, and got well of misery up fairly early. Death offers lots of thug possibilities.
With the 3 misc slots, the ML can wish without being empowered. Being immortal, he can magic duel enemy mages without fear. Having 4 death allows the summoning of banelords, and also allows the casting of higher level D spells and summons with boosters. Since the master lich is lifeless, 4D also allows him to cast drain life without carrying a D booster.
2 earth allows for summon earth power, and the earth/air combo allows the forging of either the shield of valor or the silver hauberk, one of which is a necessity for the 5 hp master lich to survive seeking arrows. 2 Air gives arcos site searches it otherwise does not have, and in combination with the nature magic, allows the forging of the coveted rainbow armor. 2 nature allows the forging of a N1 booster, so your priestess can forge N2 items such as rings of regeneration, eye shields, vine shields amulet of resilience etc.
On a big map, 3 growth is important to income when the game reaches turn 80 or so. Still need income for castles, labs, pd, etc. 3 order, even with the cold and sloth, allows for the gold and resources to expand with 20 resource cost elephants. The high resource cost infantry units are not overly favored by me due to high encumbrance, low damage weapons, and slow movement. Just need enough to mix in with the elephants to boost morale.
As soon as possible start producing elephants. Yes there is nothing quite like the sight of 50+ elephants trampling an enemy army into dust. Early game 2, 15+ mobs of elephants roaming around will fix most indies.....if your neighbor gets cheeky, just crush him under the weight of your elephants. Cost - 100 Gold 20 resources mean that you won't be pumping much more then 5 - 10 at the very most in your early game...don't worry though, as mentioned above they are pretty hardy and you don't need hundreds of them to get the job done (though it would be nice to have a few hundred).
Morale...yeah elephant morale could be better...and if they break there is the added factor that they will turn tail and trample any units behind them....so leave the path of retreat clear in the event the trunked ones flee.
Improving Morale - Strategos have banners for a reason...yes stick a Strategos or three in the middle of your elephant wedge...it boosts their morale incredibly and very rarely will they flee unless you come up against something really worthy. But the morale and low MR is an inherent weakness, though not everyone seems to take advantage of this for whatever reason e.g. nation choice, circumstance, experience etc.
Its nice to have some Priestesses following your elephant legions around... as the trunked-ones tend to accumulate afflictions via combat but also starvation tends to rear its ugly head often too.
Early Game Priorities
In low level magic, to reduce losses against independents, body ethereal is the best spell available, and all Arco's mages are able to cast it, so alteration 3 looks like the logical first objective. On the other hand, as elephants make expansion easy even without, you may find it better to develop site searching spells first (or construction for some strategies).
Thaumaturgy 2 allows fire and earth site searching spells (but you need to recruit some mystics to have level 2 in these paths and don't have a starting e/f income, it may be better to wait a little to start elemental searches). Even so, developing thaumaturgy early is still a good choice. If you have to fight an early war, Paralyze is the best spell to counter usual rush strategies (using a small number of powerful sacreds or an awake SC).
Evocation 2, for Arcane Probing allow you to search astral sites without moving your mages (but as you have S3+ mages, you may prefer manual searches with an astrologer for provinces around the capitol). Evocation also allows better combat spells (star fire at level 1 won't win battles, but at level 3 there are good spells for fire/earth mystics (magma bolts, fireball).
Conjuration 3 (Voice of Apsu) is the last possibility for site searching, but conjuration is clearly not a priority compared to thaumaturgy or evocation (in midgame you'll want to use summon earthpower/phoenix power and power of the sphere, but these spells are only useful once you have some battle magic researched),so it's better to develop it in midgame.
Construction is mostly useful as an early choice if your pretender is awake and able to forge gem-economy construction 2 items(hammer, clam, fetish) as soon you have the gems (or is a combat god needing gear, but an awake humanoid SC is rarely a good choice). If you don't have earth 3 on your pretender, you'll need to forge earth boots before dwarven hammers (=construction 4 and a good number of earth gems, two good reasons to wait a little to develop this school).
More than other nations, Arco profits of a large magic knowledge. Developing all schools (but blood) to level 3 or 4 before going further is a good idea.
Construction 4 is my priority if not developed earlier. It will also be the first school I develop to level 6 (a good option may be to research construction 6 before the other schools, especially if you have a good fire income to forge a lot of lightless lanterns).
Enchantment 4 is the second one, especially if I'm already at war, as it offers several very interesting battlefield spells (the overpowered flaming arrows you can easily cast with a communion, antimagic a must have if you fight another astral power, Astral Healing...) and allow to summon Claymen (one of the rare cost effective troops summon your mages may use -especially interesting once you have construction 6 for the second water booster).
Then there are good spells in conjuration (water search and boost spells ; summon earthpower is particularly useful for communions, as one cast with a master gives re-invigoration to all the slaves) and alteration (luck, mostly interesting once you fight enemies using mages -against indies body ethereal is sufficient- ; quickness, and the BE Curse of Stones).
Once basic battle magic is researched, it's time to go for construction 6, allowing to forge starshine skullcaps, lightless lanterns and the multi-path boosters. But I don't think it's a good idea to continue directly to artifacts with Arco, as it would slow the discovery of endgame battle spells.
Researching blood 1 is a very good idea if you have access to blood amazons or other indie casters. Equipped with a booster (you can use a ring of sorcery) or empowered to level 2 one of them can join your communions as master and cast re-invigoration (ideally in round 5) suppressing fatigue for all the slaves, and you just need to use 2 slaves per battle for this. Give him a ranged weapon and use a script like : sabbath master, fire x3, reinvigoration, fire so he won't use his slaves for other purposes.
Then the schools to develop are thaumaturgy, evocation and alteration. Thaumaturgy 6 give you enslave mind, the spell to have if you fight a nation with low MR powerful units (Caelum mammoths, Machaka hunter spiders etc...).
If it's not the case it looks better to develop evocation first, offering at level 5 a large choice of non resistible aoe spells to spam in your communions (falling frost/fire/gifts of heaven/stellar cascades), and at level 6 astral fire and mind hunt.
Once you have both level 5 thaumaturgy and level 6 evocation, your S4 mages become terrible mind hunters (equip some of them with eye of the void, spell focus, a skullcap, a rune smasher, and kill as many enemy commanders each turn). Of course you need to be sure your enemy don't have astral mages where you cast the spell to avoid feeblemind (but as you have healers it's not an as big problem as with other nations). Mind hunt is especially good to catch spies spreading unrest in your realm.
Alteration offers the best endgame defensive battlefield enchantments (will of the fates, army of gold), and I find better to go straight for them than to stay with the weaker versions at alteration 6 or 7 (Marble warriors, battle fortune). Being able to cast will of the fates in decisive battles far before endgame, when the enemies don't have such a powerful battle magic yet, may even be worth rushing for it (skipping level 6 thaumaturgy/evocation), especially if you have found some indie air mages (and forged a crystal matrix for one of them). Armies of lucky golden fog warriors are terrific, especially combined with chained earthquakes (or rain of stones if you have a way to cast them). And with alteration 9 you also get arcane domination and wish.
Another option is to concentrate on thaumaturgy instead of alteration, and go straight to level 9, to have Master Enslave as soon as possible, when the enemies still use big armies of national troops. The spell isn't hard to cast even with rather small teams of astrologers (see Army Stealing later), but on offense loses a lot of it's value as your opponents may counter it with antimagic (and at this stage of the game any serious opponent will have some S mages to cast it). It's why I usually prefer to go for will of the fates first, it's the only battle-winning spell with no counter at all, and useful against both big armies and teams of thugs/SCs or mages.
Construction 7/Evocation 8 may be a good secondary objective, to make a team of golems and use astral tempest with them, but I'm not really convinced it's interesting to base a strategy on this. In the endgame, valuable enemies have enough MR or enough hp to fear this spell, and golems are not hard to kill with other thugs.
If you are the leader in research at this point, construction 8 is a logical priority, there may be some artifacts remaining (but don't hope to have a lot with my plan).
Astral offers very interesting endgame spells in both thaumaturgy (master enslave), enchantment (arcane nexus) and alteration (arcane domination, wish), and you don't need an astral pretender to cast them (communions allow to use any battle spell, and one empowerment on a S5 astrologer is all you may need for rituals, as you can forge ring of sorcery/wizardry and the two S boosters).
Evocation offers new terrible weapons your mystics may use in a communion (the level 9 fire and water large AoE spells). Conjuration allow you to summon abominations (with a S3/W1 astrologer and 2 boosters for each path), and probably tartarians (death magic is easy to develop even from scratch : empower someone to D1, give him a ring of sorcery, forge a skull staff, summon spectres, once you get a D3 random give him the skull staff, forge a skullface, add the 2 rings, and you have a D7 tartarian summoner).
Finally, if you have a good pearls income, wish allows you to develop blood easily (wish for magic power or for one of the blood uniques to have a powerful caster, for blood slaves if you don't have indie blood hunters).
You really want to research Soul Slay and Mind Burn early...then mind hunt with your Astrologers ASAP. Probably 1/3 to 1/2 of astrologers will be A4 right off the bat meaning you can be doing a lot of mind hunting in the right circumstances (ie. enemy doesn't also have astral mages). Keep 2 or 3 Priestesses handy to heal the feebleminded ones ASAP..its remarkable how quickly they do it too if they are scripted to heal every turn. Example....say you know you are about to face a large enemy army that has a fair number of death mages....hopefully you can scry the army before it gets to you... anyway if the enemy army is free of astral mages you can pretty ,much go to town on them with your A4 mindhunting astrologers. Try and take them down before they hurtesses your precious elephantses.
I like to slowly upgrade one or even better two Astrologers up to A8 or A9 so that spells like Arcane Nexus and Wish are able to be cast....mmmm....Wish = SC of your choice. I like Seraphs with nice gear....others like Doom horrors (can backfire though....not nice)
Another tactic that works is while your elephant legions are off conquering....or whatever and need some magical backup, teleport as many of these guys as you can spare...20 is nice....sit them at the edge of the battlefield and script them to soul slay. It whittles down the enemy nicely.
In a big fight, casting Doom ( curse the entire enemy army) followed by Will of the fates (Luck for your whole army)...nice when you have a giant elephant legion with luck.
Baalz said: I find Mystics to be quite underrated. They're recruitable anywhere and communions of them can rain down falling frost/fire, gifts from heaven, magma eruption, blade wind, astral fires, enslave mind, and cleansing water depending on what the situation merits plus most any buff (legions of steel/strength of giants/weapons of sharpness turn their quite respectable heavy infantry into a major threat). You can also use them to forge slave matrixes for thugs, then buff them with every earth/fire/water/astral buff through a communion (invulnerability, fire shield & breath of winter turns most anybody into a thug, toss in quicken self, personal luck, and ). They make an awesome defensive force while you're building up your research base and a damn effective offensive force when you collect critical mass.
Silhouette said: I think Arco priestesses are under-used in communions. Make a handfull of crystal and slave matrices and setup the communion as normal, and use slave mx on some more cheap priestesses to enlarge the communion. Instant flexibility -- you can switch them to masters with the crystal mx instead and turn the communion upside down so it can lead with multiple H5+ casters for a surprise Banish/Smite/Maggots party vs undead. Even one H4+ master in communion for Fanaticism and Divine bless can be worth including full time in the team, plus the nature path buffs for slaves such as Eagle Eyes, Elemental Fortitude, Personal Regen, etc. At higher research levels, simple priestess masters have pretty big guns like Charm, Relief, Mass Regen/Protection, Howl.
Your astral mages are already powerful and can be boosted in round one with light of the northern star. It allows Arcocephale to use it's own style of communions, using the "reverse communion" tip (see the guide to communions) but with far more masters : slaves boosted by power of the spheres and light of the northern star spam low level astral spells, and masters the classic elemental ae.
You need to be lucky with mages ID to use this with maximum efficiency : powerful astral mages should be in the beginning of the list and mages good in elemental paths in the end (astrologers are the best slaves and mystics the best masters). The first mage should be out of communion, just here to cast "light of the northern star" in round one, so all mages take less fatigue even for the communion spell. If possible two of the masters should have a crystal matrix one to cast "power of the spheres" and the second for "summon earthpower", so all slaves get reinvigoration and one more level since round one. Then all slaves cast low level 20 fatigue spells like "stellar cascades", "paralyze" (or even "soul slay" for the ones starting with S4 or more) and masters level 3 evocations like falling frost/fires. If you want to use high fatigue spells like will of the fates when you use this tactic, cast them out of communion, it's better to have one uncounscious astrologer than to tire and finally lose many slaves.
Of course this kind of communion increase a little the risk of losing some slaves, and valuable astrologers instead of expandable mystics. The IA may go mad in round 6 and use any big spell the slaves can cast, so I can't say this tactic is 100% safe ; but usually, 12 slaves and 8 master survive about 15 rounds without problem. If you fear a longer fight, you can add some slaves after the masters, just here to divide more fatigue (but avoid to give pearls to the casting slaves to reduce fatigue, the IA loves to make them cast spells like Astral Healing or Doom in round 6).
What is the common point between astral, earth, water and nature ? All these schools have spells doing stun damage, and Arco can easily cast all of them (with alteration 4; thaumaturgy 4, evocation 5).
So an excellent battle plan is to combine them. Use many mages spamming stellar cascades (all the slaves in an half reverse communion), as communion master an earth mage with penetration items casting Curse of Stones (if possible with a matrix to have the spell active since round one), and three water mages (far from each other so they don't cast on the same targets) spamming Curse of the Desert. Add some priestesses with thistle maces or matrices casting Sleep Cloud the first rounds (if the enemies are living being).... Half of the opponents should be unconscious before reaching your troops (of course ordered to "hold & attack") and the others so tired that most hits will be critical.
In late game you can even add Heat from Hell or Grip of Winter to the sauce (once you have enchantment 8 to protect your own troops with a Warriors of Muspellheim/Niefelheim + Relief) and if you are cruel kill all enemies with fatigue only (use all your troops as mages bodyguards and watch the opponents fall).
Opening a Second Front
Teleport allow you surprise attack with teams of astrologers in the middle of an enemy realm. Teleported teams of mages can be a nightmare for your opponents, especially on big maps.
As you have water, your mages can be equipped with bottles of living water at construction 6. The mages with bottles will be scripted to cast body ethereal in first round, often affecting their elementals. Include in the team some S3/F1 to cast astral fires, and some S4 enslaving enemies (so you'll end with some troops) ; if possible give a penetration item to all (as they don't need precision for their spells, don't fear to use an eye of the void) and some extra gems (especially fire or other elemental gems, so if the fight isn't won in 5 rounds the fire astrologers will summon will o the wisps or elementals). Use another astrologer, equipped with a bow and scripted to fire, to carry a gem provision (including the astral pearls you may need for a returning).
5 or 6 well geared astrologers (half having bottles) usually win against any kind or level of PD. And you can prepare them with a ritual of returning and a light armor to survive the first hit if you fear to encounter too much opposition.
Once you have taken a province, two possibilities : there are numerous enemies around or not. If opposition looks too strong for your mages, script returning in first round (or a vortex of returning if you can cast it) to be sure to save your team, and try another province the turn after. If you think the mages can survive one turn (eventually with the help of some PD) script them for combat, build a lab and start to recruit indie commanders. Use the enslaved troops as bodyguards for the astrologers without bottles if the enemy can use assassination spells.
The next turn you can gateway an army in the middle of ennemy territory (equip some S4 astrologers with leadership items so they can carry about 60 soldiers + many magical beings eventually) and really open a second front. Split the troops between the indie commanders and other mages, so you can use the leadership gear for a second wave. The only problem may be the lack of holy magic (as you can't teleport a priest). Summon telestic animates if you think you need it (then if you can keep the province some turns, build a temple and a fort and recruit priestesses).
Master Enslave is a risky spell to cast with medium sized communions, especially if some slaves are weak ; the high level mean many will take extra fatigue, and if the fight is long, the master(s) continuing to cast will kill some.
It's finally safer to use Master Enslave with small communions of good mages (in a magic 0 or more dominion you only need 6 astrologers : one to cast light of the northern star in the beginning of the fight, 4 slaves who must have S3 to survive, one S5 master with a matrix to cast master enslave in round one). In a magic 0 dominion, the master take exactly 200 fatigue and won't be wounded ; the 4 slaves take just a little more, losing 4 or 5 hp, but shouldn't be killed ; if you are in magic 2+ dominion or use one more slave, they won't even suffer a wound. As he is himself unconscious the master won't continue to cast (except if the battle is incredibly long). But any stun damage (or other dmg as any hit will probably be critical) may kill the communiants, and they will also be lost if the army is finally routed.
Even better you can steal an army without risk, if it's not a problem to lose the province, using 7 astrologers (the 5 first with S3, one S5 and a last S3), 12 pearls and a matrix, the last astrologer being scripted to cast Vortex of Returning just after the Master Enslave. As astrologers can cast teleport/gateway, it's usually not a problem to have them and their troops returned to your capitol.
As all this occur in round one there is no chance to fail if you are defending a province -and the enemy, unable to use antimagic will lose far more troops-. And, even if you are the attacker, odds are small that the enemy can counter this in one round.
Frontier Labs and Mobile Research Centers
One of the national advantages of Arcocephale is a 100gp discount on laboratories, and labs are especially useful to an astral nation, as they allow to gateway troops to their province. So, when most other nations only build labs in their forts, it's a good idea for Arco to build them in a far bigger number of provinces, especially the frontiers and strategic choke points.
Labs on frontiers allow to organize surprise offensive with gatewayed troops, but also to defend your realm far more efficiently if you use these provinces as research centers. There is no reason for researchers to stay in a fort, where they can be assassinated or targeted by spells exactly like in open ground. A lab without fort on a frontier province make your mages useful for research and defense at the same time. The only problem is : you can't at the same time protect your mages with a dome and have the lab open for gatewayed troops. So the best defensive architecture use 2 labs in adjacent provinces : one research center protected by a dome and one lab in a neighbor province to send troops. You can use the frontier as the research center, so mages + local troops stop small armies alone, or retreat to the other province where troops are gatewayed if a bigger one approach. Or let the frontier province with a lab empty, with the research center in the second one, to use an ambush tactic : when enemies approach troops are gatewayed and mages move to the frontier.
Your main armies can as well be considered as mobile research centers : if you plan to stay somewhere 2 turns, always build a lab ; and if an interesting new spell is near to be found never hesitate to stop your armies and research it. If you want to keep your rank in the research race and use huge communions with 30+ mages on the field at the same time it's the way to go. Having your battle mages researching 1 turn / 3 (one turn they take an enemy province, next they build a lab and search sites, the third they research) is an huge advantage when most enemies waste the research capacity of all the mages they use with their armies. This kind of slow progression also allow you to fight more often in defense, where your mages cast first (so spending one round to launch a communion isn't a problem). And if you happen to lose a battle, the net of labs allow you to recover far more easily, you just have to gateway troops to the previous one (probably where the survivors have retreated). At worse you'll give some labs to your opponents, but if they don't destroy them and forget to cast a dome, it can be an advantage as they make easier to open a second front (as described in 5.3).
If Arco don't have a good access to thugs, fire + water allow at construction 6 to use any commander as an artillery unit, casting 2 times more often than mages, with Boots of Quickness and a Wand of Wild Fire. According to my tests such a commander kills about 50% more units per fight (against a big army of protection 14 human heavy infantry) compared to a Bane Lord with Quickness Boots (same gem cost if items are forged with a hammer). Of course the artillery commander need an escort and won't be as useful against big monsters, enemy thugs etc... (or fire immune creatures of course) but equipping as many commanders as possible is definitively a good investment as long your enemies use normal armies (and against bigger or very high protection preys, a Rod of the Phoenix instead of the WoWF allow to kill one troll or jotun giant per round... or a bane lord in two).