The AI's play was focused in the two castle assaults that I defended. It attacked the wall at two places. The units attacking the wall didn't take many casualties, and eventually broke through. However in the second assault, the AI left it's army inside the range of the castle towers and walls, and it took a lot of casualties waiting for the walls to be broken down. The castles may have to be adjusted with non-shooting walls and fewer towers. The objective is to make a castle assault be more costly than waiting for the castle to surrender.

Dismounted units that turn into some kind of artillery would be ok. I think they could be balanced in such a way to be useful in castle assault, but not in field battles. Since the AI never dismounts, it won't be trying to use the dismounted unit in field battles. The non-dismounted form of the unit can be balanced to be useful in field battles.