[cjauvin]

Wow thank you! So I just learned the proper name of this "puzzle", which my father also showed me a long time ago (and for which I used to fill pages and pages of trials even though I knew that it had no solution), and which I tried to render as an "applet toy", a couple of years ago on my blog (see my other comment below).

[TuringTest]

It's interesting that there exists a solution (shown in the Wikipedia article), if you build the rooms on a torus. That would be the equivalent of digging a tunnel between two rooms.