Castle Gargantua, and may I just say - Bravo!
This is the kind of Old School product I love - innovative. It takes a difficult concept and makes it concrete and playable - something truly different for players (and referees) to experience. Hey, a good, old-fashioned dungeon is fun, and I think most of us get the urge to run or play in one every so often. But it's these innovative adventures that keep everything fresh.
Now, I'm not going to go into how it's done, because that would ruin the surprise for the players. Suffice to say - Castle Gargantua gets it done. I can affirm this because I tried to do something similar with the city of Dis in my Hell Crawl a while back, and KK does it better. Quite a bit better, in fact. Dang it.
1) You get moody, evocative art by Jeremy Hart and excellent maps by Dyson Logos
2) You get fantastic rumors to drive the adventurers on - I really enjoyed these
3) There is a random element to the dungeon, so there is the possibility of repetition. I think referees might need to veer from the script from time to time. A minor quibble, and it does not detract from the quality of the composition or execution, because ...
4) ... the random elements are awesome - super fun to read, and probably super fun for players to experience. Having written a few hex crawls, I can attest to the quality of the imagination in Castle Gargantua and the effort it must have taken to write.
Final thought ... buy the book. For $5.00, the PDF is a steal. I'm going to shuffle off and buy a hard copy myself.