If you have some long range artillery, it might be an idea to bring it along. I tried to defend a bridge and had a bombard in my army. I just chose the bridge in the most defensible position and placed the cannon on a hill just behind my blocking force. I was hoping to do some bowling with tightly packed enemies on the bridge. It might have been interesting for my own unit on the bridge, but unfortunately, the bombard's accuracy was on par with Scuds so only a handful enemies got hit. A higher valour unit might have been better or battlefield artillery with better accuracy might be worth a shot (Sorry, couldn't resist the pun )

Otherwise, when the AI park on a hill, you can try to irritate him a bit with a ranged unit. This works well if the enemy doesn't have missile units, otherwise they will shoot you to pieces. Try to flank the AI if there are nearby hills. It might give you the high ground and make the ai move, even if it taks some time to get in position. If this doesn't work, storm and hope your troops can pull it of.

MV