Nice map Sep, you are the go to guy for modified earth.

As I recall, all Europe could have become Mongols if not for Genghis returning home due to a single man's death. He had to go back to elect a new leader. By the time he could return to the area Europe had established a number of technologies that would have prohibited such an invasion. (Castles, Knights, etc). I'm going from lame memory so don't bother trying to quote any of this.