embraser

Embrasure comes from the Old French embraser (to widen an opening, to splay), possibly related to braser (to burn, to cut). The word entered English in the early eighteenth century, though the architectural feature is medieval. An embrasure is any opening in a fortification wall designed for firing weapons — either the gaps between merlons (the raised sections of a battlement) or the splayed window openings in tower walls.

The design of an embrasure is a study in asymmetry. The opening on the defender's side is wide — wide enough to swing a bow or aim a crossbow. The opening on the attacker's side is a narrow slit, sometimes only a few inches across. The splay channels the defender's field of fire outward while presenting the smallest possible target to the attacker. This is architecture as ballistic engineering.

Arrow loops — the narrow vertical slits in castle walls — are a specific type of embrasure. They evolved from simple vertical slits to cross-shaped openings (allowing lateral aiming) to keyhole shapes (accommodating both crossbows and early handguns). Each evolution tracked the weapons being used. When cannons became standard, embrasures widened dramatically — a cannon requires a much larger opening than a crossbow.

The word survives in modern military architecture. Gun ports in bunkers, firing positions in pillboxes, and even the gun ports on naval vessels are all embrasures. The principle has not changed since the medieval period: make the opening wide on your side and narrow on theirs.