垛口是指城墙上的矮墙（城堞）前方突起的部分。可用来作为守御城墙者在反击攻城者时的掩蔽之用。一般垛口曾数十公分到数公尺都有。

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.