Crenellated Battlement

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 rectangular gaps or indentations occur at intervals to allow for the discharge of arrows or other missiles from within the defences. These gaps are termed "crenels" (also known as carnels, embrasures, or wheelers), and a previously unbroken parapet is termed crenellation. Thus a defensive building might be designed and built with battlements, or a manor house might be fortified by adding battlements, where no parapet previously existed, or cutting crenellations into its existing parapet wall. The solid widths between the crenels are called merlons (also cops or kneelers). A wall with battlements is said to be crenelated or embattled. Battlements on walls have protected walkways (chemin de ronde) behind them. On tower or building tops, the (often flat) roof is used as the protected fighting platform.

The term originated in about the 14th century from the Old French word batailler, "to fortify with batailles" (fixed or movable turrets of defence). The word crenel derives from the ancient French cren (modern French cran), meaning a notch, mortice or other gap cut out often to receive another element or fixing. The modern French word for crenel is creneau, also used to describe a gap of any kind, for example a parking space at the side of the road between two cars, interval between groups of marching troops or a timeslot in a broadcast.

source: wikipedia