What is "murder-hole"

A murder hole or meurtrière is a hole in the ceiling of a gateway or passageway in a fortification through which the defenders could fire, throw or pour harmful substances or objects, such as rocks, arrows, scalding water, hot sand, quicklime, tar, or boiling oil, down on attackers. Boiling oil was rarely used because of its cost. Similar holes, called machicolations, were often located in the curtain walls of castles, fortified manor houses, and city walls. The parapet would project over corbels so that holes would be located over the exterior face of the wall, allowing the defenders to target attackers at the base of the wall.