Concentrating on the late medieval period, Age of Chivalry: Hegemony replaces or significantly alters each of the civilizations in the original game, while adding a number of new ones, allowing the player to control Central and Western European states. Many new units and technologies have been added and gameplay dynamics have been changed considerably in this complete overhaul of Age of Empires II.
This mod is excellent and fun to play. There are very few bugs and the graphics are beautiful. Of course it could always use more civilizations and a higher population limit, but it those are just some things I would like to see added and are not really necessary.