Arrow Slot

A wide hallway ends in a door. The door has an arrow slot in it.

Mechanical Trap

On either side of the door are concealed holes that allow a ballista to fire through them. A DC 16 Wisdom (Perception) check detects two spots on either side of the door that are slightly discolored, about 1 sq foot each. A DC 18 Wisdom (Perception) check detects that on either side of the middle of the hallway, there are two tiles in the floor which are less worn and are identical to each other.  When one of these tiles is stepped on, the corresponding ballista fires. This is a +8 ranged melee attack for 4d6 piercing and bludgeoning damage that hits the first target in a line from the concealed hole in the wall. The tiles can be disabled with a DC 14 Dexterity check with thieves’ tools. Failure causes the ballista to fire. The tiles can also be easily avoided by walking in the center of the hallway.