More on this, I had a similar problem a couple of days ago, again related to a bridge.
House Tully's bard was camping the far side of a bridge so I sent an assassin to deal with him. It appeared that he was aware of the assassin as he began to move towards him and fixed him in place right at the base of the bridge. So.. I sent a second assassin to deal with this, assuming the first had been enthralled. He also became stuck at the base of the bridge. I believe I had an assassinate cursor in this second instance, rather than an arrest the bard cursor which is what usually happens in this case.
Finally I placed an archer unit on the far side and eventually the mess was sorted. Oddly, both assassins were eventually able to move again. *shrug*
Forgive the lack of detail, it was merely noted in passing while playing a game. This was the Tully vs. Arryn map, Trident-Small. It was the bridge at the top of the map between the leftmost keep and the ruin, keep side.
Edit: gave correct map