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.
Originally the term referred to a covered passage way that traversed the ditch between the walls of a fortress and a ravelin outside the wall. This was more than simply a passage however as fire from this point could sweep the ditch between the ravelin and the curtain wall and inflict devastating damage on any attempt to storm the wall. Thus the passageway was equipped with musket ports and cannon ports that fired along the ditch.
It usually takes the form of a low blockhouse, often partly sunk into the floor of the ditch, projecting outward into the ditch with access from the main fortress via a passage through the curtain wall, or as fortresses became largely underground, via a tunnel from within the fort. The roof is vulnerable to plunging fire, and is thus usually exceedingly thick and curved to deflect falling shells, or covered with a thick layer of earth.
The caponier is usually equipped with a firing step and rifle ports to allow troops to fire along the ditch, and often has provision for small cannon to sweep the ditch as well. To clear the smoke and fumes from the firing the roof of the caponier is often provided with ventilation ports.
To avoid fire from one caponier bearing on the next, caponiers are usually set at alternate corners of the fort, so that they fire towards a blank wall at the opposite end of the ditch, giving full coverage of the ditch without subjecting the next caponier to fire. The length of the straight sections of the ditch is chosen so that it can be covered by fire from a single caponier. Caponiers are often wedge shaped so that they can fire down both angles of the ditch.
Counterscarp battery 
An alternative to the caponier is a counterscarp battery, dug into the outer face of the corner of the ditch, giving a similar field of fire. Again reached by a tunnel from within the fort, it does not have the vulnerable roof that the caponier has, but being outside the ditch, is potentially more vulnerable to mining. Both structures may be found in the same fort.
Locations of caponiers 
Caponiers are a common feature of 19th century fortification, and are found on almost all the Victorian forts of Malta, the Palmerston Forts in UK, the Lisbon Entrenched Camp forts in Portugal, fortifications in many Nordic countries, and in:
- Brest-Litovsk fortress,
- Kiev fortress and
- Sevastopol (Ukraine).
- Craignethan Castle in Scotland contains a good 16th century example.
- Boden Fortress in Sweden
- Southsea Castle
- Spandau Citadel
- Suomenlinna in Finland
- Fort Washington, Maryland, USA
- Fort Hamilton, New York City, USA
- Coalhouse Fort, Essex, England
- York Redoubt, Halifax Regional Municipality, Canada
Caponier, Fort Napoleon, Ostend, Belgium
See also 
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