The smaller West, demi caponier was built to mount four small smooth bore guns to fire down the West ditch of the fort. This armament was later changed to two 32pr Smooth Bore Breech Loading guns. Like the other caponiers its armament was never fitted.
A tunnel runs from the west end of the barrack block to the west caponier, providing a secure means of moving troops to their fighting positions in the caponier itself, the west mortar battery and on to the west rampart. Troops whose duties were in the west mortar battery would have exited the tunnel up a spiral staircase midway along the length of the tunnel.
The west caponier is one of the areas of Fort Nelson that has not yet been restored and access is only possible on special occasions, such as Heritage Open days. The flooring, upper and lower, is missing. The gun ports are bricked up. At the end of the caponier are the expense magazines that stored the ammunition for the guns in the caponier. The caponier is protected by an upper and lower flanking gallery that provided rifle fire across the face of the caponier
The West demi-caponier at Fort Nelson.