Just had another great battle in M2:TW. Terrain CAN make a huge difference to the direction of a battle.
I was the Portuguese, and my foes were the Moors. My army was lead by Prince Affonso, who I was trying to train to be a perfect general. The Moors attacked me with a large, 3/4 stack army composed of desert cavalry, archers and spear militia. They were at the bottom of a huge (and I mean huge) hill. I, meanwhile, was at the top.
My archers and crossbows were positioned on a ridge, and could fire directly down at the enemy. Naturally, they fell like flies as the arrows bounced off their rather unprotected heads. Even soldiers what made it to the top were too tired to fight my spear militia gurading the only passage to the top of the hill. I only lost around 30 men, out of about 470.