21 SECRET PASSAGE. The passage (built into the wall) between Room 20 and the Inner Ward only allows single file.
The covered pit trap will always allow the first person to cross. Each person after that has a 3 in 6 chance of setting off the trap. It will drop them into a twenty foot deep pit. Anyone in the pit will be able to hear rushing water from the river directly below them. The cover will reset by itself in six hours.