A battlement is an extremely strong wall built to defend a city or castle from enemies while providing cover to defensive troops. The oldest known battlements were constructed in ancient Egypt.
During the Middle Ages — and for thousands of years before that — battlements were one of the main defenses during war. They could take the form of separate walls surrounding a city, or be part of a building. In either case, they featured regular gaps or windows called crenels, through which defenders could fire weapons at advancing attackers. Battlement comes from the Old French bastille, "fortress."