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.

In 2026, historians and architects primarily refer to these narrow vertical openings as arrowslits or arrow loops (also known as loopholed walls or balistraria). Developed during the medieval period, these apertures allowed archers or crossbowmen to launch projectiles at attackers while remaining almost entirely protected by the thick stone walls. The interior of the window is usually splayed wide to provide a broad field of vision and range of motion for the defender, while the exterior opening remains a slim slit to minimize the target area for enemy fire. In later centuries, these evolved into "cross-loops" to accommodate the wider arms of a crossbow or "cannon loops" for early artillery. Visiting a castle like Carcassonne or the Tower of London today, you will notice that these slits are strategically placed at various heights—often at waist level for standing archers—to cover "dead zones" where attackers might otherwise find cover.