Castiel: And the answer is the key, like the Doors of Durin in Lord of the Rings.
Referencing Lord of the Rings, a fantasy novel series by J.R.R. Tolkein (and three movies based on it) concerning the adventures of a group comprised of humans, elves, and hobbits to destroy an evil ring. To destroy the ring, it must be cast into Mount Doom. The Fellowship must pass through the Dwarven city of Khazad-dum and to enter, they must solve the riddle on the doors.