Probably the best RTS game I've ever played. The battles are really epic in size and very, very realistic which adds a lot of depth to the game. The strategy portion of the game (including diplomacy) is also very well balanced and immersive building up to a truly awesome game.
The game is long. Really long. You can easily spend 80hours on a single campaign and yet it never feels repetitive which is credit to the excellent battle engine and really elaborate turn-based strategy system. During fights, troops are brave or wimpy, weather changes in real-time, sounds are dampened in the forest, you can organize ambushes, cavalry attack can wreak havok when surprising the enemy and so on. The only thing missing is that naval battles are computer-only but that's just a tiny nitpick.
In the strategy section, you get a real feeling of the complexity of waging a world war. Resources are stretched, people happiness fluctuates, diplomacy can significantly affect outcome of various regional wars, you can easily mass a large portion of your armies in one area and leave another one open, and so on. The buildings vary by faction although they don't have that much of a difference in practice. Each faction gets some unique-looking building but they all do very similar things which is a bit of a downer. For example, you may conquer a barbarian city which has a temple to some obscure local god but ... you can just as easily leave it in place and not convert it to a roman temple because the effect is still the same (say 5% boost in "happiness")
But once again, this is basically nit-picking. The game is absolutely awesome with great visuals, excellent sound and exquisite depth. One of those rare games you can still play a few years later.