Battlements — Middle Ages

The story behind the answer

Those tooth-like notches form a crenellated fighting line atop a wall. The solid uprights are merlons and the gaps are crenels; together with the wall-walk they make the full defensive feature in question. This distinguishes it from a simple parapet (a plain protective wall) and from machicolations (stone projections with holes to drop missiles).

Medieval defenders used the merlons for cover and shot arrows or crossbows through the crenels, advancing in a rhythm of shelter and fire. Such crenellations spread across Europe from late antiquity, becoming symbols of authority as well as military utility. In later centuries, they even turned decorative, crowning Gothic Revival buildings to evoke the power and romance of castles.