The Conquerors adds five new civilizations, including two from the americas, the Incas and Aztecs. It also adds eleven new units such as the halberdier (an answer to the powerful paladins) and eagle warriors, which are used by the Incas and Aztecs instead of stable units; a unique technology for each civilization including Kataparuto for the Japanese (increases Trebuchet fire rate and pack/unpack speed) and Zealotry for the Saracens (increases hit points for camels and mamelukes); and several new general technologies, such as Bloodlines, which increases the hit points of mounted units, and thumbring, which increases the rate of fire and accuracy of archers.
Several game balancing tweaks have been added too, for example janissaries have had their attack increased, and mangonels don't auto attack if some of your units could be hurt.One of the biggest changes, however, is the villager AI has been changed so they now gather after making a drop off building...i.e. after making a lumber camp they will start to chop wood.
AOK:TC also adds three new types of play, such as Wonder Race in which the goal is to build a wonder before anyone else.