A fortified gun emplacement or covered passage in a fort or military fortification that allows defenders to fire on attackers.
From French caponnière, possibly related to capon (the bird), though the military meaning's origin is uncertain. Some scholars suggest it may relate to a defensive position shaped like an enclosed space.
Military architects used the term 'caponniere' across Europe for centuries, and it's one of those words that shows how French military terminology dominated European fortress-building!
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