"A merlon is the solid upright section of a battlement (a crenellated parapet) in medieval architecture or fortifications. Merlons are sometimes pierced by narrow, vertical embrasures ...
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or slits designed for observation and fire. The space between two merlons is called a crenel, and a succession of merlons and crenels is a crenellation. Crenels designed in later eras for use by cannons were also called embrasures." - (en.wikipedia.org 08.05.2022)