While not explicitly a dungeon feature, the ceiling is a part of the game where certain monsters or traps can be located. The player rarely interacts with the ceiling, and some special levels (such as some Quest home levels or the Plane of Air) do not have one since they are considered outside.
Currently, there are only five monsters that use the ceiling.
Piercers and the lurker above hide on the ceiling until something stumbles onto their square. A hard helm will protect the player from a piercer's damage, but beware that pets will be instakilled by lurkers. When trying to move onto a pet wumpus, it will not be displaced into water and drown because their feet will stick to the ceiling.
When stepping on a falling rock trap, the game states that a rock falls from a trapdoor on the ceiling (even if the ceiling is air.)
A drum of earthquake, when improvised, will unhide any monsters hidden on the ceiling.
Applying a mirror up will produce the message You reflect the ceiling.
Quaffing a cursed potion of gain level causes you to "rise through the ceiling" unless you are on dungeon level 1 without the amulet, the endgame levels, Sokoban, the top floors of the Wizard's and Vlad's tower, the top floor of the Quest, and Fort Ludios.
Reading a scroll of earth produces the message The ceiling rumbles around you! and creates several boulders.
There is a 1/4 chance that you may trigger a special type of trap door when kicking a throne. The message when this happens is, "The floor opens up under you!" followed by "The hole in the ceiling above you closes up." This is the only method of triggering this trap.