Narrow vertical opening, normally in the walls or battlements of a castle. Often built with deeply splayed inner jambs for firing arrows. Also known as loopholes or arrowslits.
In Canterbury most arrow loops in the city wall and its bastions, and in Westgate tower, have been converted at some stage to gun loops (q.v).
Local examples (click to enlarge)
Cemetery Gate, St Augustine's Abbey, Monastery Street
Bastion, Pound Lane