These are one of the defensive constructs that will be appearing in Moria:
Defensive doors consisting of a great circle of granite designed to roll in front of a narrow tunnel entrance. The entry is sunken so the stone is held vertically in a trough. It overlaps the frame of the door and can be wedged in place, making it nearly impossible to roll aside from the outside. From the interior it is quite easy to move.
A variant of this door features a narrow hole bored through the center of the stone, allowing a steel rod to be pushed through. One end of the rod features a "T" that unfolds flat against the stone's surface, the other end is held in place with a metal pin. This allows a bar to be fastened across the stone, held in place by the rod and the walls of the tunnel.
The locking version of this door is often used to create cul-de-sac style traps. The enemy is led into a large chamber with multiple looping corridors leading back to the entry area. The pursued party loops through the corridors, exits and seals the door, trapping the pursuers within. Water or poison gas can be used to kill the enemy, or they can simply be left to starve.