Battles, archers and muskets on walls seem to increase chances of CTD

I have experienced many CTDs in city battles and over time have narrowed down the criteria which seem to increase the chances of CTD. Namely, the presence of archers on the walls and/or a great number of units on the walls engaged in combat.

In this regard, I think perhaps there is an ongoing bug in the Total War series, because in Medieval Total War I experienced 100% certain crashes when using musket units (guns, can't remember their official name) on the walls. Whenever I placed a musket unit on a wall and fired down into, say, a courtyard at ground level - BOOM, CTD.

Has anyone else experienced this? I submit my observations in case persons more experienced than me at programming may be able to figure out the cause.

Cheers
OB

This sounds unreasonable. I have certainly used ranged units on walls, without getting any crashes from it.