Arrowslit

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

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 variety of forms. A common one is the cross, accommodating the use of both the longbow and the crossbow. The narrow vertical aperture permits the archer large degrees of freedom to vary the elevation and direction of their bowshot but makes it difficult for attackers to harm the archer since there is only a small target at which to aim.

Balistraria, plural balistrariae, from balister, crossbowman can often be found in the curtain walls of medieval battlements beneath the crenellations.