Door Jams. Within five or ten feet of certain distinguishable doorways in this section of the dungeon are pressure plates in the stone floor. When a weight of at least 100 pounds presses down on the pressure plate, a great slab of iron* drops downward from the ceiling with a grating shriek and seals the doorway. The slab remains in place until there is less than 100 pounds of weight upon the pressure plate at which point the slab retracts swiftly into the ceiling.
A pressure plate can be disabled once its inner workings have been deduced given effort, time, and the appropriate tools. A disabled pressure plate will not cause the slab to drop if it is already in the ceiling or will not raise it if it is already blocking the doorway. Failure in the attempt results in the great pressures of the mechanism in the floor and walls to erupt into an explosion of stone and iron. The pressure plate is disabled, but each creature in a 20-foot-radius of the pressure plate must make a DC 15 Dexterity saving throw. A target takes take 4d10 piercing damage on a failed save, or half as much damage on a successful one. The area becomes difficult terrain.
The numerous denizens of this level of the dungeon have learned to use these doorways both offensively and defensively in their constant struggle over dungeon turf. They say there's a gauntlet of twenty of them at a choke point in the dungeon, a long blood-soaked corridor cast in hellish magical darkness. There in the doorways lurk eight ropers under the thrall of Lord Villainous which have been transmogrified to look like a pile of discolored bricks instead of a cave formation. They also have devil's sight.
* Slabs of iron have AC 19, HP 50, and damage threshold 10. They are immune to poison and psychic and resistant to cold and fire. Trying to bash down a slab of iron is a noisy affair, drawing unwanted attention from all over this level of the dungeon.