A murder-hole is a hole in the ceiling of a gateway or passageway in a fortification through which the defenders can fire, throw or pour dangerous or noxious substances at attackers. As a result, the defenders would be able to rain rocks, arrows, boiling oil, molten lead and other substances down on the attackers heads.
Similar holes, called machicolations, were often located in the curtain walls of castles and city walls. The parapet would project over corbels so that holes would be located over the exterior face of the wall, and arrows could be shot at, or boiling water or oil poured over, any attackers near the wall.
