A loophole, a word from the Latin sagitta,[1]​ (also called arquera, saetera, lancera, embrace or ballestera) is a vertical, narrow and deep opening, made in some walls or defensive walls, as well as in the towers of castles or even in some battlements, to allow shooting arrows with bows "Bow (weapon)") or with crossbows.[1][2]​.

The reason that gave rise to the loopholes was the intention to eliminate blind spots in the fortifications. In this way, the first female archers appeared in the fortresses of the 19th century, although at that time they were still quite rare or difficult to find. It was during the century when they multiplied and became a common defensive method. Towards the middle of the century, loopholes became increasingly rare in the lower parts of the fortifications and multiplied in the higher areas. As a general rule, the name "loophole" is reserved for the oldest fortifications in which there were no firearms, and loophole for those in which these types of weapons were used to fire. For example, the loopholes for rifle shooting in the polygonal forts of the century.

In naval artillery of the centuries, embrasures were the openings in the sides, generally square openings, through which warships fired their cannons.