When GameSpy shut down last year, a large number of games lost multiplayer support thanks to their reliance on the service. One of these games was Civilization 3, and since the closure the game has gone without any kind of multiplayer. Those days are finally over though, thanks to a new patch from 2K that uses Steam to enable multiplayer.
The system “functions identically” according to the Steam update notes for Civilization 3, although Direct IP connections are no longer supported.
As Civilization 3 is a game from back in the days when most people bought games on physical discs, many players won’t have Civ3 in their Steam library. In this case, a support request sent to 2K will help you out, as the publisher can send you over the patch files.