A battlement, in defensive architecture, such as that of city walls or castles, comprises a parapet (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 embrasures, also known as crenels or crenelles, and a wall or building with them is described as crenellated; alternative older terms are castellated and embattled. The act of adding crenels to a previously unbroken parapet is termed crenellation.

The function of battlements in war is to protect the defenders by giving them part of the parapet to hide behind, from which they can quickly expose themselves to launch projectiles, then retreat behind the parapet. 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. A distinctive feature of late medieval English church architecture is to crenellate the tops of church towers, and often the tops of lower walls. These are essentially decorative rather than functional, as are many examples on secular buildings.

The solid widths between the crenels are called merlons. Battlements on walls have protected walkways, termed chemin de ronde behind them. On tower or building tops, the often flat roof is used as a protected fighting platform.