This key can be found in a dead body in a cave behind Mad Mage Quest room. There's a rope spot west from chests where you get the rewards. Go up, then south. Go down to a room with three Cyclops and down again to a small cave with some Rats. The corpse is located in the south-east corner under some rubbish.
Key 3667 opens the door to a room, over Minotaur Outcasts' camp, where Cyclops guard Key 3610 and some other treasures. There's a lever that allows access to the same room by moving part of the wall but it also closes the way back and can only be used once every Floor Reset.