Civilization IV has Multiplayer support from the outset. Firaxis already had a working version of MP in the Fall of 2003 and throughout development Firaxians played 4+ hour MP games in their own time on Wednesday nights. For Civ4, Firaxis went ahead and designed a multiplayer game first, which has paid off dividends for the single-player game. "We got all the rules set using the multiplayer engine, and then by using that, we learned what players would do. And that helped us write the AI," Firaxis president Jeff Briggs said.
There are many different forms of MP available in Civ4. Players are able to play everything from quick games of a few hours -- online with the Fast game mode and simultaneous turns -- to protracted games that are stretched out over months -- turn-based PBEM with the Marathon game mode. The forms of MP that Civ4 supports are: LAN, Internet, PBEM (Play-By-Email), Hotseat and Pitboss. It will support both simultaneous turns and classic turn-based mode. GameSpy will be used for Internet match-making. An improved random map generator ensures equal starting positions for all players. The server browser in the game will include buddy list functionality to keep track of your friends easier. Meanwhile, Firaxis will cull all sorts of statistics from multiplayer matches, which will let it rank players, which is aimed to avoid some of the frustration when you find yourself in a multiplayer match with ruthless Civ pros.
Pitboss is a new form of MP for Civ: it's a a persistent turn-based server (PTBS -> Pitboss) which tracks players' turns all the time, even if they are not connected to it. Players can hop on for a live session and then carry it over to a PBEM game based on the schedules of everyone involved. Basically, Pitboss will let you play a game of Civ online like normal. However, when someone has to leave (and usually someone has to leave over the course of a 10-hour game of Civ), Pitboss will save the state of the game on its server. Then you can log in every now and then to see if your turn has come up. If it does, you can make your moves and save the progress to the server for the next player. This will keep a multiplayer game going, sort of like Play-By-Email. And when everyone is back online at the same time, you can resume the game at full speed. You can participate in multiple Pitboss games at a time. So if one game is proceeding slowly, you can start another to keep you interested.
Co-op mode is available for Multiplayer, which works in a similar fashion as in RTS games. Here several players are part of a locked Alliance in which everything is shared: line of sight, Wonder effects, research (it's even possible to for two Civs in an alliance to research the same Advance to get it twice as fast) and territory. Units can be traded. Victory in this mode is also shared: if one of the players wins, his/her allies win as well. Note: earlier claims that co-op mode allows several players take control of the same Civ are false -- every player still has his own Civ.
Civilization IV supports up to 12 players in Multiplayer game.