CAPONIER (from the Fr. caponnière, properly a capon-cote or house), in fortification, a work constructed in the ditch of a fort. Its fire (musketry, machine-guns, case shot, etc.) sweeps the bottom of the ditch and prevents an enemy from establishing himself in it. The term is used in a military sense as early as in the late 17th century. In various bastioned systems of fortification a caponier served merely as a covered means of access to outworks, the bastion trace providing for the defence of the ditch by fire from the main parapet.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)