Kickplate

An iron kickplate is on the bottom of the door.

Mechanical Trap

The kickplate is spring-loaded and slams down on the target’s feet if the door is opened. A DC 12 Intelligence (Investigation) check reveals that the kick plate, is attached to a mechanical device attached to the door. When the door is opened, the target must make a DC 12 Dexterity saving throw. On a failure, the target takes 1d6 bludgeoning damage and is restrained. The target’s foot can be removed by pulling out their foot with a DC 10 Strength (Athletics) check, causing 1d4 slashing damage, or by lifting up the plate with a DC 12 Strength (Athletics) check. The trap can be disarmed by removing the plate. This requires a DC 12 Dexterity check with thieves’ tools and takes 10 minutes.