crenelation

Crenelation is the pattern of alternating raised sections and gaps along the top of a castle wall or tower—the classic notched silhouette of medieval fortifications.

The gaps allowed defenders to shoot while the raised portions provided cover.

The word describes both the architectural feature and the process of adding it.

Crenelated walls appear in everything from genuine castles to decorative architecture mimicking the medieval style: The mansion's crenelated roofline gave it a fortress-like appearance.