Fort Brockhurst was one of a series of forts built around Portsmouth in the nineteenth century as part of the Gosport Advanced Line, a series of fortifications built to protect the dockyards from a potential French invasion further up the Hampshire coast. Although it was never needed for this purpose, the fort remained in military use until 1957 and today we have a postcard of some members of the Royal Field Artillery at the fort in the First World War:
The gunners are wearing white drill fatigue uniforms, with soft peaked caps:
Behind the men can be seen a field gun, its shield and barrel just visible over the men:
The fort would act as a recruitment and then a demobilisation depot during the Great War, was damaged by bombing in the Second World War and passed to the Department of the Environment in 1957 before ownership passing to English Heritage in the 1980s. It is now open to the public as a visitor attraction.
Tales from the Supply Depot 
Interesting picture. I believe that the rivet detail on the recoil mechanism beneath the barrel identifies the field gun as a German 10cm K 17. These fired a 105mm shell and were first produced in 1917. The building in the right and centre background still stands in the middle of the parade ground. In the left background can be seen the fort itself with one gun port visible to the left of the main entrance tunnel (mostly hidden by the neck of the rear left soldier).