For flank defence of the main branches of the ditch, casemated buildings that projected into the ditch were constructed to mount short range guns. These guns were specially adapted to fire case shot only. At Fort Nelson there were two smaller, demi caponiers with guns to fire down the ditches of the flanks and one full, or double, caponier to defend the two major branches of the main North ditch. When the fort was competed the armament for the caponiers was undecided. It was not until the early 1880s that a special gun and carriage was adopted for the flank defence of ditches. This was the 32pr. Smooth Bore Breech loading gun of 42 cwt.
Fort Nelson was to be armed with four of these guns in the main north caponier and two in each of the demi caponiers. Musketry loopholes were placed in side galleries to flank the faces of the caponiers and a sally port on the upper floor of these flanking galleries allowed troops to access the ditch in order to clear obstructions under cover of darkness. Caponiers were a short lived feature of the Palmerston forts due to their vulnerability to plunging shot and mining.
The main (north) caponier at Fort Nelson.