Civ 4 continues the series tradition by making minor but important changes to the core game formula.
The first, and most obvious, change is the graphics engine. Civ 4 marks the first game in the series to use a 3d engine. While this has little effect on the gameplay in a turn-based strategy games such as this, it manages to improve the overall look and feel of the game. One of my favorite graphical touches in civ 4 is the world map. When you first start the game your map is very limited to the world around you. As you explore, you're field of view will expand but the map remains flat. Finally when you discover a key technology (I think it's magnetism) the zoomed out version of the world map changes to a globe rather than just a flat map.
Another graphical change this time around is that cities take up more and more of their borders as they grow rather than just the center tile as in previous versions of the game. Again, this is a minor change, but it always irked me a little in previous games that a bustling metropolis was represented as the same size as a small village on the map.
The UI has aslo been ravamped and provides a much more user-friendly and seemless experience than in the previous games. Just about any detail you need to know can be obtained simply by mousing over an item.
The second major change to the game is the addition of religion to the gameplay. The primary benenfits of religion are the effect they have on your culture, expanded view of the world map depending on what cities chare your religion, and the happiness of your populations. The extent of these effects is determined by whether or not you founded the religion and how you choose to manage your religion (ie Theocracy vs. Free Religion) rather than what particular religion you choose.
The addition of religion to the gameplay coincides with the breaking up of the government aspect of previous games into five different "civics". These civics allow you greater control over how your civilization will govern itself. For instance you can choose a democratic theocracy with a socialist economy.
Another change that has been made is the expansion of the "great leaders". In Civ III you would occasionally recieve a great leader who could do things like create and army, start a golden age, or finish a building. In Civ IV there are all sorts of great leaders: great engineers, great prophets, great artists, great scientists, great merchants, etc... Each one has it's own unique abilities (for instance great merchants can create trade routes that net huge amounts of gold). The great leaders in Civ 4 appear far more often than in Civ 3 and are well implemented. When managed well the great leaders can make huge changes in the way the game is played.
Throughout the game there are various other gameplay tweaks that have been made that, overall, improve the experience. However, there are a few minor complaints that I have with the game. My biggest one is the industrial age. In previous Civ games, the start of the industrial revolution marked a huge turning point in the game, as it did in real life. The shift is much more gradual in Civ 4. Buildings, such as forges, act a precursors to the factory and the factory isn't as effective of a city improvement as in earlier games.
My other complaint is that the Civlopedia in Civ 4 is harder to access and browse through than in Civ 3. While, again, this is a pretty minor complaint, the Civlopedia is a pretty important item in a game as deep as this and was invaluable in previous versions.
I've focused my review mostly on the changes that Civ 4 has made over the previous versions, so if you have't played any of the other games you might not have gotten much out of this yet. That's alright though because there's really only one thing you need to know if you've never played Civ before: you have to play this game! The Civilization series is, by far, the greatest turn-based strategy series out there and Civ 4 is easily the best one yet. If you've never played these games before, do yourself a favor and go out and get one right now.