The ceiling here is 7 feet high.
|A tripwire runs across the hallway. Just beyond the tripwire is a 10’x10′ pressure plate.
A DC 10 Wisdom (Perception) check spots the tripwire. A DC 14 Wisdom (Perception) check reveals a 10’x10′ outline in the floor beyond. A DC 16 Wisdom (Perception) check shows a cleverly hidden outline of a small trap door in the ceiling above the pressure plate.
The tripwire does nothing. However, if not spotted, a creature must make a DC 10 Dexterity saving throw while walking over it or fall prone into the pressure plate just beyond it.
Once the pressure plate is stepped on, a large hammer swings down from the ceiling. The height of the head is 4 feet off of the floor. A creature must make a DC 14 Dexterity saving throw or be struck by the hammer, taking 3d6 bludgeoning damage. A prone target (or any target under 4 feet) still sets off the hammer, but the hammer misses.
The tripwire can be “disabled” with a DC 10 Dexterity check by cutting it. A DC 18 Intelligence (Investigation) check reveals that the tripwire is not connected to anything but the inside of the wall.
The pressure plate can be disabled with a DC 14 Dexterity check, but requires a small shim or similar object to wedge into it.
The ceiling trap door can likewise be disabled with a DC 14 Dexterity check, and requires a spike, nail, or other object to nail or wedge shut.
The denizens of this lair kept forgetting about the hammer trap and placed the tripwire in place. They either saw the tripwire, and were reminded of the trap, or tripped over it, and missing the hammer blow.