Embrasure In military architecture , an embrasure is the opening in a crenellation or battlement between the two raised solid portions or merlon s , sometimes called a " crenel " or " crenelle " . In domestic architecture this refers to the outward splay of a window or arrow slit on the inside . A loophole , arrow loop or arrow slit passes through a solid wall and was originally for use by archers . The purpose of embrasures is to allow weapons to be fired out from the fortification while the firer remains under cover . The splay of the wall on the inside provides room for the soldier and his equipment , and allows them to get as close to the wall face and arrow slit itself as possible . Excellent examples of deep embrasures with arrow slits are to be seen at Aigues - Mortes and Château de Coucy , both in France . History . The etymology of " embrasure " expresses " widening " . The invention of the arrowslit is attributed to Archimedes during the siege of Syracuse in 214 – 212 BC . From Polybius ' s ( c . 200 – 118 BC ) " The Histories " : [ Archimedes ] had had the walls pierced with large numbers of loopholes at the height of a man , which were about a palm ' s breadth wide at the outer surface of the walls . Behind each of these and inside the walls were stationed archers with rows of so - called " scorpions " , a small catapult which discharged iron darts , and by shooting through these embrasures they put many of the marines out of action . However the invention was later forgotten until reintroduced in the 12th century . See arrowslit . By the 19th century , a distinction was made between embrasures being used for cannon , and loopholes being used for musket ry . In both cases , the opening was normally made wider on the inside of the wall than the outside . The outside was made as narrow as possible ( slightly wider than the muzzle of the weapon intended to use it ) so as to afford the most difficult possible shot to attackers firing back , but the inside had to be wider in order to enable the weapon to be swiveled around so as to aim over a reasonably large arc . Variations . A distinction was made between horizontal and vertical embrasures or loopholes , depending on the orientation of the slit formed in the outside wall . Vertical loopholes — which are much more common — allow the weapon to be easily raised and lowered in elevation so as to cover a variety of ranges easily . However to sweep from side to side the weapon ( and its firer or crew ) must bodily move from side to side to pivot around the muzzle , which is effectively fixed by the slit . Horizontal loopholes , on the other hand , facilitate quick sweeping across the arc in front , but make large adjustments in elevation very difficult . They were usually used in circumstances where the range was very restricted anyway , or where rapid cover of a wide field of arc was preferred . Another variation had both horizontal and vertical slits arranged in the form of a cross , and was called a " crosslet loop " or an " arbalestina " since it was principally intended for arbalest iers ( crossbowmen ) . In the sixteenth and seventeenth centuries , after the crossbow had become obsolete as a military weapon , crosslet loopholes were still sometimes created as a decorative architectural feature with a Christian symbolism . A stepped embrasure was often utilized on pillbox bunkers of the 20th century , allowing for a relatively wide field of fire compared to a traditional embrasure while also minimizing the shot trap phenomenon created by the sloped opening . A series of perpendicular " steps " tapering to the gun port ensured that any incoming fire would be stopped by a vertical impact and not funneled inward towards the slit .