Arrow slit

An arrowslit (often also referred to as an arrow loop, loophole or loop hole, and sometimes a balistraria) is a thin vertical aperture in a fortification through which an archer can launch arrows.

The interior walls behind an arrow loop are often cut away at an oblique angle so that the archer has a wide field of view and field of fire. Arrow slits come in a remarkable variety. A common and recognizable form is the cross. The thin vertical aperture permits the archer large degrees of freedom to vary the elevation and direction of his bowshot but makes it difficult for attackers to harm the archer since there is only a small target at which to aim.

Balistraria can often be found in the curtain walls of medieval battlements beneath the crenellations.

The invention of the arrowslit is attributed to Archimedes during the siege of Syracuse in 214–212 BC. Slits "of the height of a man and about a palm's width on the outside" allowed defenders to shoot bows and scorpions (an ancient siege engine) from within the city walls. Although used in late Greek and Roman defences, arrowslits were not present in early Norman castles. They are only reintroduced to military architecture towards the end of the 12th century, with the castles of Dover and Framlingham in England, and Richard the Lionheart's Château Gaillard in France. In these early examples, arrowslits were positioned to protect sections of the castle wall, rather than all sides of the castle. In the 13th century, it became common for arrowslits to be placed all around a castle's defences.

...
Wikipedia