A projecting parapet supported by corbels on a medieval castle; has openings through which stones or boiling water could be dropped on an enemy WordNet 3.0 & Open English WordNet

A machicolation is a floor opening between the supporting corbels of a battlement, through which stones or other material, such as boiling water, hot sand, quicklime or boiling cooking oil, could be dropped on attackers at the base of a defensive wall. Wikipedia

An opening between corbels that support a projecting parapet, or in the floor of a gallery or the roof of a portal, of a fortified building from which missiles can be shot or heated items dropped upon assailants attacking the base of the walls. Wiktionary

Opening between corbels that support a projecting parapet, or in the floor of a gallery or the roof of a portal, of a fortified building. Wiktionary (translation)