Door knockers have been popular since antiquity. Some have abstract or purely functional designs, but many are figurative.
Common motifs include foxes, fish, lions, or fantastical beasts. Lion door knockers typically hold a ring in their mouths, with the mouth used as a hinge, and the ring striking against a plate in the door.
Somewhere deep in the dungeon is a door that can't be opened by any obvious mundane or magical means. The door bears the figurative brass head of a beast with a gaping mouth.
The door will only open if its matching brass ring (hidden elsewhere in the dungeon) is inserted into the beast's mouth, and used to knock on the door.
The thing on the other side of the door is the reason someone went to the trouble of removing the brass ring and locking the door in this manner. Beware!
(This is for the A-Z Blogger Challenge.)
Thanks to Richard Kelland, Tim Whitlow, and Ilaria Caterina for creating the Creative Commons photos used in this post.