A narrow vertical slit in a castle wall or tower from which an archer can launch arrows or bolts from a crossbow at attackers. The slit was thin, making it near impossible for attackers on the outside to hit an archer behind the wall as they would have to get their arrows to go through the slit.

The narrow vertical openings found in castle walls, formally known as arrow slits, "embrasures," or "loopholes," were ingenious defensive features designed to provide archers and crossbowmen with a tactical advantage during a siege. From the outside, the slits are extremely thin—often only a few inches wide—making it nearly impossible for an enemy attacker to land a successful shot through the gap. However, on the inside, the wall is hollowed out into a wider, fan-shaped recess called an "alcove." This configuration allowed the defender to stand close to the opening and swing their bow horizontally or vertically, significantly increasing their field of vision and range of fire while remaining almost entirely protected by the thick stone masonry. As gunpowder became more prevalent, these slits evolved into circular "gun loops" to accommodate the barrels of early firearms. Beyond their military utility, these openings also provided essential ventilation and a small amount of natural light into the otherwise dark, damp corridors of the castle's defensive perimeter.