There seem to be a number of prims on the sims around Orientation Castle, most notably in the Frisch (southwest corner) and Duerrenmatt sims. (Half of a house, complete with fires, is at Duerrenematt 144,67,304, and a treehouse is at 185,216,107)
The areas are marked as no build, but either these were there when it was marked, or someone(s) found a way around it...