murder hole

Short definition from Wiktionary

Noun

1. 1
   A slit or hole above the entrance to a fortification, thought to provide access for defenders to kill invaders.
2. 2
   A chamber above the entrance to a fortification where defenders hide and shoot at invaders trapped in the entryway below.
3. 3
   Alternative form of murder hole.
4. 4
   A deep pit used for drowning malefactors as a punishment.
5. 5
   A waterhole or pit in a bog where the bodies of murder victims were thrown.