Question

and then knock knock! i hear again. is it aunt barb from baltimore? what if she brought mean uncle glen? i cant—and wont—answer the door. again knock knock! please make it stop. it could be an evil centaur. was that the sound of hooves—clop clop? who knows whats on above behind that closed door?

Explanation:

The context is about someone being afraid of who or what is on the other side of a closed door. The preposition 'behind' is the most appropriate as it refers to what is on the other side in a spatial - relationship related to a door.

Answer:

behind