Yeah, I've cleared the dungeon area. Unless there's some secret entrance/exit that's not on the map, I've seen everything. The dungeon is just one big loop w/ doors at either end leading to the lower levels of the fortress. There are no doors on the tower itself--none that I can see, at least, and none indicated on the map.
"...Blame this on the misfortune of your birth." -Char Aznable
I've done some exploring to check this for you. The entrance to the tower is level with the keep's walls. you have access to the top of that wall through another tower, which is guarded by a locked door. Climb up that tower and it'll lead you where you need to be.