A battlement in defensive architecture, such as that of city walls or castles, comprises a parapet (i.e., a defensive low wall between chest-height and head-height), 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. These gaps are termed "crenels" (also known as carnels, or embrasures), and a wall or building with them is called crenellated; alternative (older) terms are castellated and embattled. The act of adding crenels to a previously unbroken parapet is termed crenellation.