In addition to the difficulties of the lemming soldiers, I have also noticed sometimes the AI gets 'stuck' on the bridge. Several times now I have seen the tail-end of the attacking enemy army (3-4 full units) all pile up on their end of the bridge and stop there while the rest of it moves forward. Even after all other units are dead, those units sit there and don't move. If I charge into them, it takes a good 10-15 seconds for them to react and fight back. Until that time, they sit still and take casualties without noticing my men. After they 'wake up' the whole formation shifts as if it had just come unstuck and is now able to move freely.