A crenellation (also called a battlement) is a defensive parapet on a castle wall featuring an alternating pattern of solid blocks called merlons and rectangular gaps called crenels. It allowed defenders to safely shoot at enemies through the gaps while hiding behind the solid stone blocks for cover.

Key Components of Crenellations:
Merlons: The solid, upright sections of stone (typically 4–5 feet wide and 3–7 feet high) that shield defenders from incoming arrows and stones.

Crenels (Embrasures): The open, notched gaps (typically 2–3 feet wide) through which archers and defenders step forward to aim and launch projectiles.

Arrow-loops: Narrow vertical slits sometimes carved directly into the center of the merlons, allowing soldiers to fire without exposing themselves in the open crenel.

Historical and Architectural Evolution:
Strategic Defense: Positioned along the top of curtain walls and gatehouses to protect the open-air wall-walk (chemin de ronde) where the garrison moved during a siege.

Licence to Crenellate: In medieval England and Wales, lords needed official royal permission called a licence to crenellate to legally add these military features to their properties, signaling both defense and status.

Symbolic Transition: As medieval warfare shifted toward gunpowder and cannons, crenellations lost their pure military utility and became decorative status symbols on manor houses, stately homes, and Gothic Revival buildings.