War[edit]
The key to war in Civilization Revolution is to follow the rule of overwhelming force. Armies are the only way to go for attacking and defending. Spies are another important aspect, you need them to destroy enemy fortifications before you attack with an army of catapults. The best invasion force early on are two catapult armies, a knight army and a spy. Placing a galleon in the port of the city you are attacking will also make a difference.
Very early on in the game, a warrior army accompanied by a horseman unit can take a weak civ like the Mongols, but it is a better strategy to wait until another civ has built smaller cities for you to take.
Later on in the game, multiple cannon armies offer the biggest bang for the buck (pun intended). And, if you haven't won by the time artillery is invented, artillery armies are the only way to go.
Be aware that the AI for Civ Rev is more random than Civ 3 or 4. You have a 1/3 chance of winning with evenly matched forces and a 3/4 chance with more powerful forces.
Also, don't forget that you can weaken a city by kidnapping its Great People - which is fun to do. Also, by dominating a choke point on a hill near the city intended for destruction, you can slowly kill off the enemy (provided they are on a plain or desert area - this can be wickedly fun, especially if you group your warriors from the beginning of the game into an army.
Don't forget the power and advantage you get from placing a ship in the port and don't ever sit there in front of a city waiting for more units to arrive, the AI will just suck the population out of a city and make unit after defensive unit.
Defense[edit]
As far as defense goes. Archer armies, Rifleman armies and Modern Infantry armies will do the trick. If you have a city that is going to be attacked over and over, place at leas two armies there. For higher levels, you need two armies in each city. If you know you are going to be attacked over and over, do not group some units into armies, and instead leave them as they are. This will make it take longer for the enemy to be able to take the city.
Early on, catapults can be used to demolish attacking forces. Do not fortify them in the city, and instead make them wait so they will be less likely to be attacked - the same goes for artillery and cannons.
Planes and bombers are pretty useless unless you find yourself in a nasty battle. And, walls are useless for the most part, when you can spend your time building a huge army to counter-strike against your enemy, or destroy the invasion force.