[n] a rampart built around the top of a castle with regular gaps for firing arrows or guns
crenelation, crenellation
abatis, advanced work, balistraria, bank, banquette, barbed-wire entanglement, barbican, barricade, barrier, bartizan, bastion, breastwork, bulwark, casemate, castellation, cheval-de-frise, circumvallation, contravallation, counterscarp, crenel, curtain,