Murder-hole

Immediately inside the doorway is a small lobby which is defended by a cross loop which would have been manned from inside the main chamber and defended by a murder-hole over.

Source: wiktionary

The murder-hole above the outer end of the passage survives, but only the descending grooves mark the position of the outer portcullis within it, beneath the recess to receive it in the outer E. wall

Source: wiktionary

This corridor is entered from the Castle by a tunnel cut through the rampart, its exit through the wall being guarded by a murder-hole above, and made more secure by a double right-angle bend and a narrow doorway.

Source: wiktionary

Marcus caught up another large rock, but before hurling it he paused, squinting eagerly down his murder-hole.

Source: wiktionary