A small peephole is in the door.
|The peephole is actually a small blowgun. It is powered by a small air bladder that sits underneath the tile directly in front of the door.
A DC 14 Wisdom (Perception) check reveals that the tile just in front of the door is slightly raised compared to neighboring tiles.
A DC 16 Wisdom (Perception) check shows that no light comes through the peephole from the other side. Closer inspection is required to actually see that there is a small bamboo dart in the blowhole. A DC 18 Wisdom (Perception) check is required to see that the end of the dart is slightly moistened with a clear liquid.
When the door is approached and the tile stepped on, it pushes air into the door and peephole which fires the dart. The peephole is placed 5 ft above the floor and will strike anyone within 15′ of the door.
The dart is a +4 ranged attack for 1d2 damage, and the stricken creature must make a DC 14 Constitution saving throw become poisoned. After each long rest, the poisoned creature may attempt another DC 14 Constitution saving throw to end the effect. Otherwise, the effect may be ended by a protection from poison spell or similar effect.
If noticed, the door can be accessed by stepping around the tile with a successful DC 12 Dexterity (Acrobatics) check.
The peephole can be plugged with any small piece of wood, metal or cloth and requires a successful DC 12 Intelligence (Investigation) or Dexterity check.
The tile can be lifted with a successful DC 14 Dexterity check, and then punctured with any sharp object.