This is the sixth level of the six-level Hobby Shop Dungeon, created by Ernie Gygax and Terry Kuntz:
Comparing it to my maps of Felltower, this is a big level. My bigger levels are on 11 x 17 paper with 8 squares to the inch. This one is 22 x 17 and it seems like it's close to my 8 squares and inch.
That's a tough map to use, for me, though. 11 x 17 is already hard enough for me to keep where I can see it but others can't. And the thin walls scream "D&D" to me because GURPS players would just tear the walls down with Shape Earth spells if they're that thin.
But it's very cool to see, and it's fun to have this to compare with my maps. I like the amount of unkeyed space, too - lots to wander through, making wandering monster rolls and finding little but combat you don't really want. The sheer amount of walking you'd need to do means you'd need magical light sources, plenty of time, and just patience to keep moving around looking for the actual prizes.