A series of open notches and raised sections (merlons) along the top of a castle wall or fortification.
From 'crenelate' plus the noun-forming suffix '-ation.' This term describes the architectural feature itself rather than the action of creating it, standardized in English by the 16th century.
The crenelation design was so smart for medieval warfare that it's been copied for 1,000 years—you'll see it on everything from toy castles to modern government buildings trying to look 'official and serious'!
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