There are other pages on the internet that list strategies for the different achievements. There are a few good tips to keep in mind, too. What I'll list here is what I would suggest to use against the hardest to complete.
Complete by 1000AD on king or higher
this is definitely one of the hardest in the game. here is a tactic that sometimes works, but note that the position of everyone is everything. it can decide if you finish before 2000BC or even meet the target 1000AD. scenario: it's money that matters.
civilization: arabs (due to fundamentalism, they get +1 to attack which is helpful, but if you REALLY don't want to use this civilization, i recommend any other war based civilization or the aztecs with instant healing, but i do recommend arabs.)
difficulty: king obviously, but some people find this a good way of getting the deity domination achievement, but i would recommend one or the other.
turn 1: begin by converting your main city to produce as much science as possible. a good coastal town does save you a turn or two, considering you only have approx 49 turns to win. immediately research horseback riding. NOTE: every turn, you MUST use the mass of gold you have to immediately complete a warrior, until you get horseback riding. then do the same with horsemen. using the warriors you produce, send them out in random directions looking for other civs and barbarians but DO NOT get noticed by these civs where possible (they dont explore much) and DO NOT attack the barbarians (you will see why later)
after a few turns you will have horseback riding. now, keep rushing these until you have an army. looking at where the other civs are and the barbarians, make this horsemen army attack barbarians to get it to ATLEAST elite status (6 battles) and produce another horsemen army at this time. getting scouting and infiltration are helpful upgrades. if you can get them, try to. they will help.
your next research should be towards mathematics. this means you will need to research the alphabet, pottery, writing, literacy, masonry and then mathematics. mathematics will give you catapults. so once you get mathematics, start producing catapults instead of horsemen.
head off to the first Civ and hopefully using a few (3 is good) horsemen armies, you will beat them. you will earn all their gold aswell. build a road, and then repeat for the next civ. if you can pick up any catapults now, it is helpful. now attempt at the second civ, and you will have 3 capitals. when i did this, two were on the same continent as me and two were on island. if you are lucky, they will all be within close proximity of one another and if not (e.g they are all on islands or you are on an island) id recommend restarting.
by building up lots and lots of catapults and horsemen, you should be able to innhialate any capitals. TIPS: spies are very useful for lowering defense of capitals, especially the last one. a spy ring is more hopeful, as it is less likely to be combatted by another spy. navigation may be useful to research after you have got mathematics, you wont have time for the next major unit upgrade before 1000AD. you must complete the game BEFORE 1000AD, so you can't complete it on that day. often, if you come in contact with the 3rd civ you are conquering, the likelyhood is they will know the 4th. a good thing to do is accept the their offer of peace and then see if you can arrange a meeting. there you go! you now know where the last civ is. remember, this will not always work. a good starting point is critical. often if you can conquer the first civ, it will work. remember to save after every conquer and you should do fine after a few tries. good luck!
Win by domination on the hardest difficulty
I've seen other people suggest strategies for winning this, but I think this can be done easier than anything I've read online.
Basically, you only need to take the capitol cities. That's only 4 cities. They're also almost always coastal. If this is played on the Beta Centauri scenario, you don't need to worry about scientific advancement. This is where the computer players really have an edge (time). I like Aztecs because they are never wounded (unless you retreat).
The gist is:
- Develop an inexhaustible economy.
Focus on money at first. Expand to just a few close cities. Build Market and Bank.
- Scale-up an effectively unlimited manufacturing potential.
Once the cashflow is established, invest in production. Also, food and culture.
- Assemble a single strike force and conquer each capitol.
Load up a dozen Modern Infantry armies and a few dozen Artillery armies (and maybe a whole mess of spies for fun) on a fleet of Battleships and head for the first city. Take a fleet escort or two if you want. At each city, park your Battleship unit and its payload both on adjacent squares and pound the city until it falls. Should only take a few turns. Pick up all but an MI or two (to occupy) and move on to the next.
- Win the game, gain the achievement.
Take the rest of the day off.
A few tips:
- Part of it is staying away from neighbors. They will tend not to bug you if they don't know you're there. It may even be worthwhile to keep your cities a little bit back from the water so a curious boat won't make introductions.
- Encountering one early might not be a dealbreaker. But keep them boxed up if you can, until you're ready to push them aside. On Beta Centauri, the barbarians can even serve to separate them from you. If you do meet stay friendly early.
- If you are getting harassed before you're ready to move out, pay others to go to war. Hopefully, it won't come to that. This might happen after you've knocked a couple of guys out already, and its obvious you want to take over the world. You can still pay them off to go to war to stall until you wipe them all out.
- Of course, this is all assuming you do keep up the normal defenses. Modern Infantry are great. Also keep Fighters. This includes the captured cities; rush new MI every turn. If you already have the funds, this should be no problem, even if everyone is on you, which might not even happen. Keep up the borders. Of course, everything is armies and wings and fleets, because you should have the production lines to support it.
- These aren't absolute rules; be a little flexible. If you plop your city down in a ring of hills, build a workshop before the bank. If spies are blowing stuff up, set up some spies at home. And if someone threatens to nuke you, build the SDI(?) defense.
What not to do:
- Take other cities. I don't know why other people insist on carefully seizing every city along the way to the capitol. You don't win any different; only the capitols count. Supply lines? They're on a boat. No supply lines. It doesn't hurt, but it's time consuming and a distraction. There may be a strategic reason to take a few, but it's not the objective.
- Tanks. Tanks are cool, but they don't have the offensive capability of Artillery or the defensive capability of Modern Infantry. They are mobile, but in this plan your units shouldn't be moving except on or off the boat. In a normal game they can be very useful but not so much here.
- Bombers. Great offensive power, but they probably won't be effective against all 4 cities (because you won't have the supply lines, ie, other cities). The same production into Artillery would go a lot further. The attack should be fast and hard. Don't sprinkling a few bombs overhead before the fleet arrives thinking you'll soften them up. You'll more likely give them enough warning and info about your offenses to let them harden their defenses. Bombing a city after the ships are ashore isn't bad if it didn't take away from building the original stack.
- Expand all over. You need money, and that should be no problem once you're attacking cities. Expanding doesn't really get you much more. It doesn't hurt, but it's another distraction if you're trying to beat this scenario.