To be honest, I haven't built a tower yet to check what can and can't be done with it. But it would be interesting if we could add locks to tower doors. Possibly a different type of lock so it could be bashable, but I don't think that's extremely necessary since the tower is quite expensive and isn't trivial to abuse. This kind of lock should only block movement from the outside of course, not from the inside.
Since we're at it, maybe consider bashable locks, with soak, also for cellars, mansions, and great halls.
An unintended consequence of that is that maybe we'll start seeing towns that look like towns, with streets and houses side by side, and not a maze of separate walled plots.