111 SHIFTING WALLS. In every corridor leading to this crossroads, at a space a little over twenty feet away, there is a broad pressure plate that shifts the walls at the crossroads.
The result of tripping the plate is to move the walls so that the path is clear for the traveler. There is a 90% chance of tripping the plate if it is not specifically found and avoided. If the plate is not activated there is a 50% chance that it is already clear for the travelers and a 50% chance that the way is blocked.
The mechanism for moving the walls is ingenious and very quiet, involving wheels and track marks on the floor. Dwarves who are looking specifically for shifting walls have a 4 in 6 chance of realizing the nature of the mechanism. It would be possible to test the shifting walls by leaving an object in the way, blocking the walls.
If the adventurers have dungeon-dwelling allies from this level, there is a good chance they know about the shifting walls, but they may or may not comment about it.