Ok, time for a final bunch of screenshots.
This is Tatar pedia, to give you the idea of differences between civs. Some, like Tatars, have several different features, some, like Hungary, a bunch of special units. I've given all features a subjective point value to achieve some kind of balance. Everything is explained in the civilopedia and compared in an Excel spreadsheet. This is of course coupled with differences coming from religion (determines available techs, units, wonders, buildings, governments etc.) and culture/location (knights vs. horse archers etc.)
So, Tatars get the Stirrup at start (not available to others as a starting tech); a fast, "nomadic" settler, Keshik, and Siege Engineer - the only artillery in the mod with 2 movement points. Tribal Council = early barracks (normally unavailable until Military Drill in the Late Middle Ages). Invasion of Europe becomes available around the time of the actual Mongol Invasion and auto-produces a Tumen (a unit of 10,000 warriors, in game-terms, almost as good as an army) until expired. The disadvantages are needed for some balance, as it's probably the easiest civ to play with the Conquest Victory as a goal.