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 the act of adding crenels to 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.