4 FALSE DOOR TRAP. After passing through the thick wooden northern door the characters will see a twenty foot long corridor ending in a second door (which is false). If the characters examine the southern aspect of the northern door, they might notice dozens of small holes.
If they touch the false door southern door (by listening, trying to open, and so on), the first door locks itself, sprouts short but sharp spikes, and rapidly moves southward.
The characters will have one full combat round to defeat the door. They may either force it, destroy it, or unlock it, but the only have time for one option. Attempts to force it open by an “Open Doors” check might succeed, but only one roll should be allowed in the time available, and the individual will suffer 1d6 damage in the attempt.
If they use weapons on the door, it can take 20 hit points of damage before being hacked down. Unlocking might be performed by thieves or a magic-user with a knock spell. If the door is not defeated, it will crush the characters for 1d10+4 each, but then may be opened. Th trap resets itself after one hour, assuming it has not been destroyed.