[@Regitnui]
Ah, gotcha, gotcha. 

What I had in mind was that the fortress moved around on legs and sort of circled the city in a random-ish patrol route that the guy doesn't have much control over. The gimmick was that he doesn't really have control of it in general, but the inside was kind of like a maze that he could only even vaguely navigate, but he'd always get where he wanted to be. The weakness was that anybody boarding the fortress looking for either its weakness or just a way to take it down would eventually come to a dead-end door with a key-hole. Any key would fit, and if you unlocked it the whole thing would just disappear.

Although, I do get why it'd be a problem, so I won't have a problem with finding something else.