I like how you implement the reinforcements, sounds like a pretty cool idea. Have you thought about putting some neutral Angmar buildings behind the wall to fill the open space? I. e. some wolf pits or barracks would fit the fortress theme, you'd just have to make sure they don't allow you to recruit units. You could also add some sort of hierarchy by putting a sorcerer building (don't know the english name, sorry^^) and/or barracks at the raised levels and maybe some orc buildings at the lower levels.
Also, are you going to add some stuff outside of the fortress or is that part finished?
Overall it looks pretty sweet already, although I would like some sort of alternative route to get into/out of the fortress, not just the main gate.