An arrowslit (often also referred to as an arrow loop 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 to aim at.
Balistraria can often be found in the curtain walls of medieval battlements beneath the crenellations.