Battlement

n. wall with openings through which weapons can be fired

A battlement, (also called a crenellation) 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).
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Noun
1. a rampart built around the top of a castle with regular gaps for firing arrows or guns
(synonym) crenelation, crenellation
(hypernym) rampart, bulwark, wall
(part-holonym) fortress, fort
(part-meronym) crenel, crenelle

(n.)
pl. The whole parapet, consisting of alternate solids and open spaces. At first purely a military feature, afterwards copied on a smaller scale with decorative features, as for churches.
  

(n.)
One of the solid upright parts of a parapet in ancient fortifications.
  

A parapet or crenellations in a castle , helpful in protecting the defenders.