I was defending my castle in a "standard" fight and I did fairly well as I managed to rout the enemy with mostly my archers and some close-quarter infantry.
The real kicker of this fight was the fact that when some of the enemy units shattered they retreated..THROUGH the castle. They actually climbed up the wall, ran across the castle courtyard and climbed down the other side in order to reach the other side of the map and the "safe point". Of course most didnt make it as they were cut down by my infantry.
This was the first time I've seen this behaviour as I usually do auto-play on sieges, but it was very cool . Needless to say there werent alot left of the besieging army afterwards due to this little retreat-fiasco.