I have been working on this MASSIVE fortress for GM mode for over the past week. I call it Fort Hope. It has a check point for magisters to check people's identities across the bridge from the entrance, a garden, a magister's barracks, a chapel and, at the very top of the mountain it is built upon, a prison. Well, the building you see at the top is just the entrance to the prison. The prison itself is built deep inside of the mountain and holds my island's most dangerous sourcerers.
It is still a work in progress and looks absolutely gorgeous in game. The below screenshots from the editor do not do it justice at all, but nonetheless, feast your eyes upon what I have done so far.
GMAddon1: Adds almost every single monster and NPC from the main campaign into GM mode, plus a few large custom maps to use in GM mode.