bastillement

Battlement comes from Old French bastillement, derived from bastiller (to fortify, to build defenses), which itself comes from bastille (a fortress or fortification). The English word appeared in the 1300s, describing the specific pattern of alternating raised sections (merlons) and gaps (crenels) along the top of a defensive wall. The pattern allowed defenders to shelter behind the merlons and shoot through the crenels. It was simple, effective, and appeared independently in fortifications from China to Scotland.

The design predates the word by millennia. Assyrian palaces from the ninth century BCE show battlement patterns in carved reliefs. The walls of Babylon had them. The Great Wall of China used them. Medieval European castle builders inherited the idea through Roman military architecture, which borrowed it from earlier Mediterranean traditions. The alternating solid-and-void pattern is so logical that calling it an 'invention' overstates the case. It was a solution that occurred to everyone who built a wall and needed to fight from behind it.

In English law, the right to add battlements to a building — called 'crenellation' — required a royal license. The Licence to Crenellate was a medieval building permit that signified trust and status. A lord who received permission to crenellate his manor was being told, in effect, that the king considered him important enough to fortify. The architectural feature was a legal symbol. Unauthorized battlements were treason.

After the introduction of gunpowder made castle walls obsolete as defenses, battlements became decorative. Gothic Revival architects in the 1800s added battlements to churches, universities, and country houses that would never face a siege. The practical feature became an aesthetic one. The gaps where archers once crouched are now where pigeons sit.