I can't really see how Civ 2's a simple game. It's very, very powerful and allows plenty different styles of playing. The strategies you can use're near endless, plus it takes very complicated algorithms to get the AI as right as in Civ 2.
Civ 3's just as simple as Civ 2, meaning not at all, but both play and feel very intuitive (which's probably what you meant with simple. Correct me if I'm wrong there) They just made changes to some of the fundamental concepts. It did result in a game that was inferior to its predecessor, and had its share of bugs, probably due to production rush to get it shipped within a deadline.
Civ 4 fixes it, as it takes the good parts of the concept changes from Civ 3 and merges them with Civ 2, plus the game shows they had time to work on it, and took that time to take out all flaws and bugs, overall improving on both Civ 2 and Civ 3. Now if only they'd've put in the throne room/palace development and made wonder movies like in Civ 2, it'd be the perfect Civilization game
