I'd like to help you, but I don't know whether you already knew this:
Temujin was chosen Khan in 1196. He commanded a few tribes he had unified. With them he attacked and conquered the Kerait (who were Christians)and the Naima. By 1204, Temudjin had unified almost all mongol tribes, and he had complete dominion over today's Mongolia. In 1205, a kind of Assembly with representation from all Mongol groups, and also from some Turkish tribes, recognized his leadership, giving him the title of Genghis Khan (Great Khan). He then organized the army, making a unified entity from the heterogeneus tribe armies. He was ready to begin his amazing conquers. In his first campaign (1209-1209) he fought against the Hsi-Hsia Realm of the Tanguts, which would become a nice base to attack the Chin Empire.
Hope it helps.
By the way, I've made a Crusader unit which I am particularly proud of. It's never been used in any scenario. I think you may like it, and suits perfectly to a 1200 AD game.
If you're interested, take a look at it at
http://www.geocities.com/fiera_com/civ2