File:Scraesdon Fort - Caponier - Geograph.org.uk - 345243.jpg
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.
Caponiers are a common feature of 19th century fortification, and are found on almost all the Victorian forts of Malta and the Palmerston Forts in UK - like this one.