(n.) A narrow opening, often cruciform, through which arrows might be discharged.

advanced work balistraria bank banquette barbed-wire entanglement barbican barricade barrier bartizan bastion battlement breastwork bulwark casemate cheval-de-frise circumvallation contravallation counterscarp curtain demibastion dike drawbridge earthwork enclosure entanglement escarp escarpment fence fieldwork fortalice fortification glacis loophole lunette machicolation mantelet merlon mound outwork palisade parados parapet portcullis postern gate rampart ravelin redan redoubt sally port scarp sconce stockade tenaille vallation vallum work