Here's a strategy that worked pretty well for me. One thing I've noticed about this game is that at the higher levels (Nobel and above) you really need a specific strategy to beat the game, at least if you are as poor a player as I am. I have never once been able to keep up with the AI in terms of research, and by about half-way through I am too far behind the AI players to keep up. When they get gunpowder, you either need to have gunpowder or overwhelming force. Fortunately, overwhelming force seems to work VERY well against the AI, as they tend to not move a lot of troops around to reinforce their borders, unless you are hovering right on their border.
Anyway, this is my strategy. Start as rome. I did it on a small map, this might not work as well on a big one. Build your first city, and immediately build a worker if its 8 to build, or a warrior if 15. Then build the other one next. Send your first warrior around to explore *manually*. Always stick to the woods for protection. If possible, allow a few wolves to attack you in the woods to build up your XP. If not possible, allow lions. After a couple battles, up the character with the woodsman skills (extra defense in woods and jungles). The second bump will give you enhanced movement which helps you scout. You need to scout a good 10-15 squares around your home city.
In terms of research, immediately research straight to iron working. Its only 2 techs to get the praetorian, which is a strength 8 melee unit. The key to this strategy is boatloads of praetorians. Once you get iron working, research the wheel so you can get roads.
In the mean time, you need to build barracks (so that you don't slow your population growth), and then immediately build a settler. About the time you get iron working, your settler should be about ready to go. Now, take your scouting warrior and your settler and plop them DIRECTLY on the closest iron you can find, even if its close to an enemy. The key here is iron, if you can't find one, its probably game over. Don't go next to the iron, go ON it.
Build your second city directly on the iron. You cannot afford to lose it to culture bombing, because by the time you get a settler to iron, your nearest enemy will be up to about 4-5 cities, and massively encroaching upon you. Tell your worker to build a road between your two cities. In your second city, first build a barracks.
At that point, set your cities to emphasize production, and build nothing but praetorians (and maybe a second worker). You want to build up a task force of about 6-8 praetorians in each of your cities. You will be getting XP from barracks, so you should level them up with the city attack skill - this gives you a great bonus, so your effective city attack can be upwards of 9.5 or so to start, and more once you get some XP.
Send out two task forces and start taking your nearest neighbor's cities. Make sure they are at least population 2, so you don't destroy them. If you have to wait for them to go to level 2, don't linger too close to the border or they will reinforce and make it harder. You need roughly 2x as many praetorians as they have defenders (they usually have archers and spearmen). You will almost always take the city with 2x. When you do, fortify them in the conquered city until they heal, and then send them to the next target.
You will get into an interesting cycle - each city you take will give you some gold, which you will need because you won't be able to support much research. Just do enough research to get the land improvements (farms, irrigation, etc). In every city you get, buy more praetorians, and continue. Keep building praetorians in your cities behind the front, and keep sending them onward to defend frontier cities.
After the first 2-4 cities are taken, add a couple catapaults to your task forces. Do NOT use them for collateral damage - just use them to bombard to lower city defenses. With this approach, you can fund all of your rapid expansion just on the spoils of war, and build a huge task force.
Shoot for taking out the capital city of one enemy. At this point, you probably want to expend your catapaults on collateral damage, just to make sure you can take the capital, and you probably want to converge both of your task forces on the capital to take it. Once you take the capital, you can take all of the land developments they made over time, AND all the wonders they built. Its nice to grab a capital city with the Pyramids right off the bat.
With this, I was able to beat a small map on Prince with a total loss of 16 units before I won the game!
Some downsides to this approach - you will be WAY behind in research. Deal with it, your only hope is to take over the whole landmass, or darn near all of it, and then use appropriate civics to grow your civ as much as possible very quickly. Once you have cities behind the front, and if you aren't fighting any more, switch over to building construction and so on (maybe with organized religion) but watch your bottom line on income! Check your finances also to make sure your maintenance from buildings and cities doesn't overcome you.
Another downside is that there will be times when you are down to 20% spending, or maybe less! This is because cities cost you a fortune, even if they are developed. Before you have a financial implosion, make sure you research currency so you can set your capital cities to build wealth to support your growing, conquered territories.
If all goes well, you'll hold them off long enough for your captured cities to mature, and you'll be able to compete in the research race and eventually finish them off.
Since research goes (inordinately and annoyingly) fast in CivIV, your only hope is to outnumber them. Since you are building tons of powerful, cheap praetorians, you should be able to hold any cities they try to take back, even with their gunpowder units.
Anyway, thats my strategy.
Anyway, this is my strategy. Start as rome. I did it on a small map, this might not work as well on a big one. Build your first city, and immediately build a worker if its 8 to build, or a warrior if 15. Then build the other one next. Send your first warrior around to explore *manually*. Always stick to the woods for protection. If possible, allow a few wolves to attack you in the woods to build up your XP. If not possible, allow lions. After a couple battles, up the character with the woodsman skills (extra defense in woods and jungles). The second bump will give you enhanced movement which helps you scout. You need to scout a good 10-15 squares around your home city.
In terms of research, immediately research straight to iron working. Its only 2 techs to get the praetorian, which is a strength 8 melee unit. The key to this strategy is boatloads of praetorians. Once you get iron working, research the wheel so you can get roads.
In the mean time, you need to build barracks (so that you don't slow your population growth), and then immediately build a settler. About the time you get iron working, your settler should be about ready to go. Now, take your scouting warrior and your settler and plop them DIRECTLY on the closest iron you can find, even if its close to an enemy. The key here is iron, if you can't find one, its probably game over. Don't go next to the iron, go ON it.
Build your second city directly on the iron. You cannot afford to lose it to culture bombing, because by the time you get a settler to iron, your nearest enemy will be up to about 4-5 cities, and massively encroaching upon you. Tell your worker to build a road between your two cities. In your second city, first build a barracks.
At that point, set your cities to emphasize production, and build nothing but praetorians (and maybe a second worker). You want to build up a task force of about 6-8 praetorians in each of your cities. You will be getting XP from barracks, so you should level them up with the city attack skill - this gives you a great bonus, so your effective city attack can be upwards of 9.5 or so to start, and more once you get some XP.
Send out two task forces and start taking your nearest neighbor's cities. Make sure they are at least population 2, so you don't destroy them. If you have to wait for them to go to level 2, don't linger too close to the border or they will reinforce and make it harder. You need roughly 2x as many praetorians as they have defenders (they usually have archers and spearmen). You will almost always take the city with 2x. When you do, fortify them in the conquered city until they heal, and then send them to the next target.
You will get into an interesting cycle - each city you take will give you some gold, which you will need because you won't be able to support much research. Just do enough research to get the land improvements (farms, irrigation, etc). In every city you get, buy more praetorians, and continue. Keep building praetorians in your cities behind the front, and keep sending them onward to defend frontier cities.
After the first 2-4 cities are taken, add a couple catapaults to your task forces. Do NOT use them for collateral damage - just use them to bombard to lower city defenses. With this approach, you can fund all of your rapid expansion just on the spoils of war, and build a huge task force.
Shoot for taking out the capital city of one enemy. At this point, you probably want to expend your catapaults on collateral damage, just to make sure you can take the capital, and you probably want to converge both of your task forces on the capital to take it. Once you take the capital, you can take all of the land developments they made over time, AND all the wonders they built. Its nice to grab a capital city with the Pyramids right off the bat.
With this, I was able to beat a small map on Prince with a total loss of 16 units before I won the game!
Some downsides to this approach - you will be WAY behind in research. Deal with it, your only hope is to take over the whole landmass, or darn near all of it, and then use appropriate civics to grow your civ as much as possible very quickly. Once you have cities behind the front, and if you aren't fighting any more, switch over to building construction and so on (maybe with organized religion) but watch your bottom line on income! Check your finances also to make sure your maintenance from buildings and cities doesn't overcome you.
Another downside is that there will be times when you are down to 20% spending, or maybe less! This is because cities cost you a fortune, even if they are developed. Before you have a financial implosion, make sure you research currency so you can set your capital cities to build wealth to support your growing, conquered territories.
If all goes well, you'll hold them off long enough for your captured cities to mature, and you'll be able to compete in the research race and eventually finish them off.
Since research goes (inordinately and annoyingly) fast in CivIV, your only hope is to outnumber them. Since you are building tons of powerful, cheap praetorians, you should be able to hold any cities they try to take back, even with their gunpowder units.
Anyway, thats my strategy.
Just make sure you don't lose the Iron City 
