After witnessing the demon land gate erupt through the keep, exit the keep and go through the west gate you went through earlier. Continue following the path until you get to an sloped area. At the top of the slope will be 2 battering ram held by rope. Just cut the ropes, and they will slide down (hopefully) killing the enemies at the bottom.
If they do not kill anything you will need to reload your previous save as the rams do not return to the top.