murder hole

noun3

1. •
   (historical) A slit or hole above the entrance to a fortification, thought to provide access for defenders to kill invaders.

   • She said the nearest she could recall was when one of King Stephen's sentries got drunk on shipwreck brandy and fell through the murder hole, breaking his leg. — 2013
2. •
   (rare, historical) A chamber above the entrance to a fortification where defenders hide and shoot at invaders trapped in the entryway below.

   • The archers on the ramparts, naturally, saw them plain, as did the towermaster in the 'murder hole' above the gate. — 2006
3. •
   A deep pit used for drowning malefactors as a punishment.

noun1

1. •
   A waterhole or pit in a bog where the bodies of murder victims were thrown.

   • The Murder Hole is the thing for me—that tells no tales—a single scuffle—a single plunge—and the fellow is dead and buried to your hand in a moment. — 1989