Wiktionary
n. (context military historical English) A type of fortification structure which allows firing along the bottom of a dry moat that surrounds the main fortress.
Wikipedia
A caponier is a type of fortification structure. The word originates from the French word caponnière (house).
The fire coming from the feature (e.g. muskets, machine-guns, case-shot etc.) sweeps along the bottom of the attendant ditch that constricts the movement of storming infantrymen to an enfilade alignment of greatest exposure and so prevents the enemy from establishing itself there.
In some types of bastioned fortifications, the caponier served only as a covered means of access to the outworks, with the bastion-trace allowing for the defence of the ditch by fire from the main parapets.
Usage examples of "caponier".
The caponier was a nearly perfectly preserved specimen of mid-nineteenth-century military architecture.
The massive walls of the caponier held out some of the afternoon heat, but by the same token, the air was stagnant and redolent with cloying floral perfume and dusting powder.