Well, after taking Minas Morgul, Mordor besieged me at once. Well, nice, since it is a citadel so it is not so easy retake. Well, the battle is another story. What I noticed was the Inner Castle was levitating about the landscape, as the image attached shows.
It was not possible for any units to go in between the space though, they had to use the gate. But I could not put archers on the walls either. And the battle AI got all funky and did not know where to drive their ram.
Bug?
EDIT: can a moderator please move this post to the Bug-forum? Too many tabs open, posted in the wrong place....