A quick, easy-to-use random (and I mean random) monster table; and,
A very 3d dungeon, switching levels repeatedly.
I think I hit both those points: 72 possible beasts, and a very mazelike, three-level dungeon, with room for expansion on three sides. Because of the systemless constraints of the contest, no hit dice or levels are indicated, so the mutants need to be improvised from description alone. I'm assuming 2nd or 3rd level for the "anti-druids", 1st for the believers, and 2 to 3 HD for the mutants.
This is probably a pretty dangerous dungeon, intended for characters of at least 3rd level, with plenty of hirelings.