A rampart built around the top of a castle with regular gaps for firing arrows or guns WordNet 3.0 & Open English WordNet

A battlement, in defensive architecture, such as that of city walls or castles, comprises a parapet, in which gaps or indentations, which are often rectangular, occur at intervals to allow for the launch of arrows or other projectiles from within the defences. Wikipedia

In fortification: an indented parapet, formed by a series of rising members called cops or merlons, separated by openings called crenelles or embrasures, the soldier sheltering himself behind the merlon while he fires through the embrasure or through a loophole in the battlement. Wiktionary

Indented parapet formed by a series of rising members. Wiktionary (translation)

A pattern along the top of a parapet (fortified wall), most often in the form of multiple, regular, rectangular spaces in the top of the wall, through which arrows or other weaponry may be shot, especially as used in medieval European architecture. Wiktionary

Pattern along the top of a parapet through which arrows or other weaponry may be shot. Wiktionary (translation)