Warlords contributes much more gameplay to a game with already infinite possibilities.
Civilization IV would be a very difficult game to design an expansion for. The core game has so much detail and so much packed into it that it seems tough to find enough new things to add. Civilization IV: Warlords does manage to pack even more into the game, feeling a bit like pumpkin pie on Thanksgiving: a bit too much, but entirely necessary and incredibly rewarding.
The changes to the game’s campaign are minimal. There’s the eponymous “Warlord” unit for every civilization. These units function much like the other great people in the game; they can join cities so they produce units faster and stronger, they can grant more experience to existing units, and they can group units into armies. Each civilization now has a unique building that functions much like the unique units. These buildings are upgraded versions of a regular building that grant some extra bonus over the regular building. 
Warlords introduces new civilizations to be beaten into submission. Six new adversaries rear their ugly heads: the Carthaginians, the Koreans, the Vikings, the Zulu, the Celts, and the Ottomans. A handful of new leaders have been added to existing civilizations as well. New wonders (including the fantastic Great Wall, which erects around the border of the civilization to keep out barbarians) round out the changes to the campaign mode. This may sound like a lot of new material, but it really does not change the way the game is played. That’s where the new scenarios (the real star of the show) come into play.