Puzzle maven Nob Yoshigahara offered this puzzle in the September-October 2000 issue of MIT Technology Review, attributing it to a Professor Kotani. In the 4 × 4 complex of rooms above, two of the rooms are closed. This leaves a single way to tour the remaining rooms in a series of orthogonal moves, visiting each room once and returning to the starting point.

The 12 × 12 complex below has a similarly unique solution. What is it?