A battlement, in defensive architecture such as that of city walls or castles, comprises a parapet (i.e. a short wall), in which portions have been cut out at intervals to allow the discharge of arrows or other missiles. These cut-out portions form crenels (also known as carnels, embrasures, loops or wheelers). The solid widths between the crenels are called merlons (also called cops or kneelers). Battlements often have openings between the supporting corbels, through which stones or burning objects could be dropped on attackers; these are known as machicolations. A wall with battlements is said to be crenellated or embattled.

The term originated around the 14th century from the Old French word batailler, "to fortify with batailles" (fixed or movable turrets of defence).

Battlements have been used for thousands of years; the earliest known example is in the palace at Medinet-Abu at Thebes in Egypt, which allegedly derives from Syrian fortresses. Battlements were used in the walls surrounding Assyrian towns, as shown on bas reliefs from Nimrud and elsewhere. Traces of them remain at Mycenae in Greece, and some ancient Greek vases suggest the existence of battlements.

In the battlements of the Roman city of Pompeii, additional protection derived from small internal buttresses or spur walls against which the defender might place himself so as to gain complete protection on one side. In the battlements of the middle ages the crenel comprised one-third of the width of the merlon, and the latter in addition featured a small slit. The same also appears in Italian battlements, where the merlon has much greater height and a distinctive cap. Italian military architects devised the Guelphic or swallowtail battlement, with V-shaped notches in the tops of the merlon, giving a horn-like effect.

The battlements of the Arabs had a more decorative and varied character, and continued from the 13th century onwards not so much for defensive purposes as for a crowning feature to their walls. They appear therefore in the same light as the cresting found in the Spanish renaissance. Similarly, European architects persistently used battlements as a purely decorative feature throughout the Decorated and Perpendicular periods. They not only occur on parapets but on the transoms of windows and on the tie-beams of roofs and on screens, and even on Tudor chimney-pots.

A further decorative treatment appears in the elaborate panelling of the merlons and that portion of the parapet walls rising above the cornice, by the introduction of quatrefoils and other conventional forms filled with foliage and shields.