battlement, crenelation, crenellation

a rampart built around the top of a castle with regular gaps for firing arrows or guns

• rampart, bulwark, wall — an embankment built around a space for defensive purposes

• crenel, crenelle — a notch or open space between two merlons in a crenelated battlement
• merlon — a solid section between two crenels in a crenelated battlement

• fortress, fort — a fortified defensive structure