As Rohan, I just captured Minus Morgul. A turn or two later, a large Mordor stack beseiged me, and then initiated an assault. As with other assaults, the AI sent one unit with a battering ram forward. It crossed the causeway right up to the door, and stopped. The AI did not actually start ramming the door until there was only a minute or so left in the battle--too late to capture the settlement.