An expansion to the already great game: Civilization III!
The "Conquests" are genius; tons of new stuff to play with; much-improved early game; great editor.
On the strength of a franchise, Civilization III sold millions of copies. The masses purchased and were, for the most part, underwhelmed. It was obviously still Civilization and it looked better than the fans could have hoped, but it felt a little empty. Civ III was a good game, but the fans expected more. The first expansion, Play the World, was released to scathing reviews from critics and players alike, in spite of the introduction of multi-player.
Now comes Conquests, one of the better expansions that the Civ series has ever seen. A lot of what comes in the box is, in fact, stuff that was added at the Play the World stage. But much of it is new, improved and interesting.