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Crenellation (or crenelation, also known as castellation) is the name for the distinctive pattern that frames the tops of the walls of many medieval castles, often called battlements. Crenellation most commonly takes the form of multiple, regular, rectangular spaces cut out of the top of the wall to allow defenders spaces to shoot arrows from and other spaces to hide behind full cover.

The fortifications of Château de Pierrefonds feature two levels of crenellated battlements

The technical name for an opening in crenellations is embrasure or crenel (or crenelle), while the raised portions of the wall between them are called merlons. Crenellations would have been used originally on defensive walls built around a settlement (with or without a castle), but the modern experience of them is mainly their appearance on the towers and turrets of castles and mock-castles. They can also be seen in large numbers on the Great Wall of China.

A license to crenelate was supposedly a grant that gave permission for a building to be fortified. This may have started in the Carolingian Empire as a way to control castle building to prevent local lords from becoming over mighty or too strong but, in feudal society, the license was used both by king and baron as a symbol of their status. With "few exceptions at times of turbulence, the king's right as overlord to license was a right to grant, not to refuse, permission to crenelate" (Coulson, 1982, p71) and "In reality, no feudal or sub-feudal ruler could either in law or in practice deny to his vassal the protection by self-help fortifying which he, as lord, had failed to provide." (Coulson, 1982, p97n10) It was not in reality necessary to obtain a license to crenelate to erect a fortified building. There was "very slight chance of interference by royal officials even in so intensively governed a realm as England, but a license was prestigious and could be had for the asking." (Coulson, 1982, p70). Fortifications were not restricted by law, but the cost of building and, particularly, of providing a garrison restricted true military castles to a very limited number anyway.

• Coulson, C., 1982, 'Hierarchism in Conventual Crenellation' Medieval Archaeology Vol. 26 pp69-10