It's the little things
Civilization 4 is a turn-based strategy game in the enormous Civilization series. Players take the role of a major world leader guiding their people from their fledgling start through history. The game is pseudo-historical, in that it takes familiar places and people from history but puts them together in a fictional world. I felt that Civ 4 was a pretty solid game, although it was a different experience than Civ 3.
As in other Civilization games, each civ has one or more leaders, which have a number of traits which encourage them down one strategic path or another. Each civilization has one special unit which adds character and helps differentiate the game further.
Different from other Civ games is the use of hit points for units. Each unit also has a single strength value, modified by various other stats (Archers get bonus defense on hills and in cities). Units also gain experience, and can have promotions as they level which impact their strength, and have temporarily reduced strength as they lose HP. This makes combat a bit deeper than it had been in previous games.
In a minor change from Civ 3, players are given more control over their civics. This lets you mix and match different kinds of government and economic policies, which adds more depth than previous games. The religious line of civics only affects cities where your state religion is present (except free religion, which prohibits state religions).
One major new mechanic was the addition of religion to the game. A religion is founded by the first civilization to discover certain technologies, such as Philosophy. The civilizations can then choose one of these religions as their state religion, which affects international relations. The presence of a religion in your city also allows you to build some specialized buildings, such as monasteries or temples, which impact the city.
The other major new mechanic was the addition of "Great People". Great people are able to impact the game in large ways by starting golden ages (a few really good turns), providing a large immediate boost to one of your cities, or joining a city for a smaller long term bonus. All civilizations can produce all great people, but Industrious and Philosophical ones tend to make more than most.
The game was significantly sped up from earlier games, largely to accommodate the multiplayer option in the game. A full game (500 turns) on normal speed can be played in 3 hours or less, if nobody cuts it short by conquering the world first. There are several different paths to victory, and they allow players to focus on the parts of the game they enjoy. For players seeking an epic experience, the game speed can be slowed down or sped up during game creation.
Overall, Civilization 4 was a pretty good strategy title. Its main flaw was that military conquest could become painfully tedious, boring, and ineffective as the game became more advanced, even with the improvements to control (select and order multiple units at once). With that minor reservation, this is a solid entry in the Civilization series, and has several expansions that provide more content and address some of the minor flaws the original game had.