Barbarossa 5: Barbarossa's March
Standard techniques are not really adequate in this scenario.
When the scenario starts, you have more units than you can reasonably control. Just leave a bunch of the weaker units behind as you advance. You have three main choices at this time. First, you could use a Siege Onager to cut through the trees and explore north of Constantinople. You will find that not much happens over there. Second, you could attack Constantinople immediately. Third, you could avoid Constantinople and head for Gallipoli.
There are too many random factors to say for sure how this scenario will play out in detail, but there are certain general themes. Suppose that we ignore the first option for now. The trees north of Constantinople are impassable at present, and that gives us some security. Suppose that we reject the second option because we don't want to do such a dastardly thing. So we head for Gallipoli, unless we get new information that convinces us to do something else.
The normal AoE scouting rule gets modified in this scenario: Scout aggressively, but cautiously! You have several types of units to use as potential scouts. We start by assuming that our opponents are primarily static, so trebuchets are the best scouts, because they have the longest sight range, and they are safe in almost all situations. As an important general rule in this type of situation, keep a large section of your army, all arms, close behind your scouts to respond to any developments.
Almost immediately, you encounter an enemy warship. It appears to be all alone. Bring up the monks and possess it! It is worth describing the process in some detail. One monk can do the actual converting, and the others can heal him as he takes damage. This allows him to stand right in the firing line, and has the advantage that the target unit won't lose interest. Alternatively, two monks could switch between avoiding fire and attempting the conversion. A variety of techniques could be used. The important thing is to practise them in easy situations so that you can deal with the hairy ones, such as when your three monks want to possess a whole fleet of four enemy ships without losing any of the ships, and definitely without losing any of the monks. Teutonic monks, of course, are better than any other ones because they get every upgrade. Incidentally, the way to capture the four ships is to maneuver until three of them fixate on one monk. Then a second monk lures the fourth ship away and possesses it out of sight of the first three, while the third monk is used for healing. And so on. As the ships get taken over, they run away and hide.
Your next encounter is with a House and a farm. As soon as you see them, they are attacked by four Elite Mamelukes. There is nothing you can do to prevent it. If you want to retain these buildings, the only option is to rush some units forward to distract the Mamelukes. The only units that could conceivably do it are your heavy cavalry. They are not the ideal choice by any means, because you could easily lose some of them unless you are very careful. If you have deployed your forces properly (all arms must be close behind the point), six Paladins should be in range, and with perfect use of Bait-and-Switch tactics, you can save your House. Some of your cavalry will be seriously wounded, but they get healed. The House will be badly damaged.
What you learn from this encounter is that you cannot use the scout cavalry as scouts. They would not have survived this encounter. You should also realize that four Elite Mamelukes popping out of nowhere would make short work of a trebuchet if they saw it, especially considering that you cannot repair your siege weapons.
Almost immediately afterwards you run into a Saracen picket line. Since you don't want to engage them immediately, you move closer to Constantinople. Without any warning at all, Constantinople attacks you! Depending on how, exactly, your forces are positioned when it happens, they may attack with only one unit, or up to ten heavy cavalry, nine pikemen, and three catapults. In any case, these units do not all arrive at once due to their different speeds. Fortunately, their first units probably don't attack your scout, so it should have time to run away. In the upcoming melee, you might be able to convert a few units, but don't count on it. You definitely should put your pikemen against their cavalry while avoiding catapults, heavy infantry against pikemen while avoiding catapults, and heavy cavalry against catapults while avoiding pikemen. The archers can grab whatever action they can. Definitely keep track of which of your units are being hit; they should run away and live to fight another day.
You now have a casus belli for attacking Constantinople. Perhaps you should do it right away, before they rebuild their army, but as it turns out, Constantinople never rebuilds. However, suppose we continue with the plan of heading for Gallipoli. You should probably re-set your diplomatic stance with Constantinople to neutral or even ally at this point.
It now becomes necessary to deal with the Saracen picket line. The essential point is that each enemy unit must be lured out of the picket line in isolation. Do not attack it until it is far from the picket line. Possess as many of these units as you can, but don't hesitate to kill them to protect your monks.
Eventually, you encounter a castle (and Tower). Here it gets a bit tricky, because you can't lure a castle out of the line, and you can't be perfectly sure that there aren't mobile units hiding nearby ready to leap all over your trebuchets. Be prepared to send Paladins in to dance around near the castle, because they can take a lot of punishment while having high mobility. After you destroy the castle, you are almost at the coast. (You could consider converting the Tower, but that's just a little extreme.)
When you approach Gallipoli, the normal course of action will produce a bad result. There are four Transport Ships near the Gallipoli dock that will come over to you, but if you take them now, they will most likely just get sunk immediately. If you think this is silly, then you need to avoid Gallipoli harbor for now. Instead, you head for the coast between Gallipoli and Constantinople. There are a number of Saracen warships there. You should possess these ships. Start with one that is all alone, and be prepared to abandon the attempt if his friends show up. Clear the coast slowly, and hide all your recruits until you have a substantial fleet. There is also a Saracen dock there that you should take over.
Incidentally, it now becomes clear that you can actually have an economy of sorts in this scenario, as can be seen from the equation Dock + Fishing Boat + Market = income. Monks are great. Not that the income stream is very efficient. If you think about this some more, you realize that you can actually produce all sorts of units (except villagers) in this scenario. Practically, though, you will be recruiting so many enemy and neutral units that you won't even know what to do with them.
When you have about ten warships, start scouting the sea. Be very cautious, because you have no way of repairing any of your ships. Lure any enemy ships over to your monks for assimilation, hopefully one by one. Eventually, you will be able to approach Gallipoli harbor and get your transports safely. If for any reason you do not get these transports, you will be forced to attack Constantinople and take their navy.
You should probably try to take out the entire Saracen navy. This isn't strictly necessary, because your mission is fundamentally about your army, but it could be useful and might be interesting. If you are going to deal with the Saracen navy, you need to do it now, before your monks are otherwise engaged. The Saracen ships do wander about, and while you have no way of predicting what any wandering ship will do at any point in time, you do know that they will eventually become aggressive. Bear in mind that the fleet in Constantinople is hostile to you at this time, so stay away from them.
At some point, the Saracen navy may decide to attack you en masse. Fight near your monks, and try to set them up as the prime targets. The monks may be too busy running around, avoiding fire and healing themselves, to do any converting, but in the meantime, your ships are sinking their ships unopposed. Eventually, the Saracens are down to just a few ships, and they will probably just wander around peacefully forever. Never mind that; sink them all! (Or possess them if they get near your monks, but you may not want to wait for that to happen.) There is one unit that may be worth waiting for: the only Cannon Galleon in the game, and it happens to be fully upgraded. This is definitely a nice unit to have, but it is by no means necessary, and in any case, all your ships will soon become completely irrelevant. To complete the defeat of the Saracens, wipe out their two Towers (there are several ways of doing this), and destroy or convert their second dock. Or leave it alive as a potential source of gold.
It now becomes appropriate to revisit the question of Constantinople. One course of action is to push forward and forget about the past. But it seems prudent to keep your lines of communication open, and to that end, we need to knock out Constantinople, and we need to do it now. We don't need to raze the whole city, just the castle and some walls, and maybe a few Towers. It is possible for a mobile unit to do the job while leaving the castle intact, but if we take out the castle, we will need to destroy some Towers first to clear some space for the trebuchets. The second option is straightforward, but it is worth describing the first option in some detail. Use your trebuchets to knock down the front gate of Constantinople. Now run a Paladin into the city, zig-zagging madly. (Actually, walk him in on a completely deterministic path.) Constantinople knows Ballistics, and Bombard Towers use it. Therefore you can avoid the fire of all such Towers, if you pay proper attention. The castle is trickier, because it fires more often, the arrows travel quite fast, and they affect an area. You can avoid most of the fire from the castle, but should try to keep your Paladin out of range of the castle as much as possible. As soon as you get him to a certain spot next to the Wonder, Constantinople recognizes that you are top dog here and hands over their fleet. Fifteen fully upgraded warships plus six fully upgraded transports is nothing to sneeze at, except that, at this point in the game, they are also completely irrelevant, because all naval opposition has already been eliminated, and you probably already have a comparable fleet of Saracen design. Incidentally, do not play diplomatic games with Constantinople after this unless you are prepared for the consequences.
The effort that we are putting into these naval considerations has some value. Fundamentally, you just need to get your troops across the water. However, if you have ships available to support the landings, and if you don't have to worry about enemy naval interference, the landings will proceed a lot smoother.
By the time all operations in the western part of the map are complete, or maybe well before that, you have probably started to explore the coast of Anatolia. On a small island in the SW part of the map, you recruit six fully upgraded Frankish axemen and a trebuchet. It's nice to have them, but they are unnecessary. Extract them from there ASAP. When you convert the second Saracen dock, or maybe by some other method, you recruit two monks and a monastery. They cannot be extracted at present, because they are at the top of a cliff. That cliff runs the full length of the coast of the eastern landmass, with only two breaks, both near Constantinople. There is actually a third way in, on the extreme northern map edge, but it will require knocking down a bunch of trees. There are also a number of orphan beaches that could be used for various purposes if it strikes your fancy.
The rest of the scenario is very straightforward. Possess what you can; kill everything else. Only the most unusual points will be described here.
You should have far more units by now than you will ever reasonably need. In fact, you could probably finish the scenario with just a few of your original units, plus new recruits. It is generally easier to just kill enemy units than to take them over. But how much time do you want to spend on this scenario? With a lot of extra effort, you could wind up with a really humungous force, but it is totally unnecessary. There are certain units that you should definitely not try to convert. Onagers, for example, are too dangerous because they could easily kill your monks with one shot.
On the northern beach, which we tackle first, there is only scattered opposition. The first enemy unit you encounter is a single monk. It is somewhat risky to try to capture him, but you do have three Teutonic monks and other advantages. To the NW, there is a group of cavalry archers and an Onager. Using standard tactics, kill the Onager and possess the cavalry archers. There is one Tower close to the beach, and one to the NE. You could possess them just for the heck of it. One monk does the dirty work, one monk heals him, and you have at least three monks. There are two enemy monks on the plateau at the north edge of the map. One is completely trapped by terrain, and cannot defend himself; the other has some mobility. Nevertheless, you can possess them both in absolute safety. Use your Siege Onagers to extract them. You now have 5 or 6 monks available. The ex-Saracen monks do not have your normal range, but they work just as well as your normal ones for many purposes. There are four free cavalry archer recruits near the NE Tower. You should now have 8 fully upgraded cavalry archers. Eventually, you are blocked to the NE and SE by static defenses.
On the southern beach, a mass of enemy units awaits you. This is not a problem; your navy can kill them all easily. The only "difficulty" is that you probably can't possess a lot of them, unless you really work at it. After you fight your way off the beach, you link up with the two monks at the monastery. Your push southward is blocked by some static defenses. Cliffs block you to the east, and more static defenses to the north.
Your next target should be the Turkish strongpoint in the middle of the map. It consists of a castle, some Towers, and some mobile units, including Bombard Cannons. Lure the mobile units away first as best you can. Bombard Cannons are actually pretty easy to deal with, because you can lure them from so far away, they have a low rate of fire, and the projectiles travel quite slowly. If you don't have any yet, you should definitely try to possess these cannons. Bombard Cannons are very useful because they are much more mobile than trebuchets, although their other properties are worse. If all the mobile units can be lured away from the castle, the castle is helpless against your long range artillery. Otherwise, those units will definitely come out to play when the artillery starts shooting. There may even be units inside the castle, and that is the big unknown.
You will need Bombard Cannons to deal with the castle in the extreme NE safely, because it is defended by a trebuchet (and a lot of cavalry archers). When the castle goes down, you should add the trebuchet to your collection. You will have been advised that you can recruit some Siege Onagers over there (there are 4), so be sure that you do not detect them before the castle is gone, otherwise you can kiss them good-bye.
At this point, very little opposition remains. There are two ways through the cliff line to your destination. In the north, you can smash some trees, deal with a few enemy units and a whole crew of wolves, and you are home. In the south, you can deal with some static defenses, and then a final line of static defenses. If you attack in the south, an Act of God helps you out, and then half the wall collapses if you even think about attacking it.
Historical Note: The interactions between Barbarossa's army and the Byzantines were by no means peaceful, and there were certainly motive, opportunity, and justification for the Crusaders to attack Constantinople. However, that did not happen at this time. It did happen a few years later, on a different Crusade.