1. The importance of geography in expansion needs to be increased by a lot. E.g. a European army should have virtually no chance against a reasonably well lead Bedouin army in the deserts of Arabia. Thus European conquest of North Africa should be far more difficult, almost impossible. A Cuman army would have similar difficulty against a European army in the terrain of Germany (although super-effective armies like the Mongols, could master almost every terrain).
Wait, what about the Crusades. You know, how the real life European armies invaded the middle east, were usually outnumbered, yet did quite well? Or how does this explain the original Muslim invasion of Spain?