Constructed in 1867, the D-shaped fort was intended to guard a vulnerable stretch of the River Thames against possible enemy landings during a period of tension with France.

Its seven casemates initially accommodated rifled breech loading guns, which were replaced by the turn of the century by more powerful breech-loaders on disappearing carriages, mounted in concrete wing batteries on either side of the fort.

Britain's coastal defences had not been substantially upgraded since the Napoleonic Wars, but a new generation of accurate and powerful guns, mounted on fast-moving, manoeuvrable iron-clad warships, had rendered obsolete the existing 18th and early 19th century forts along the British coastline.

It was the only point along a fourteen-mile stretch of the river between Cliffe to the west and Grain to the east where a landing could be effected, due to the marshes along the rest of the shoreline.

[3] As an 1869 report noted, the fort was constructed "to prevent an enemy landing at the only accessible point for the purpose of attacking Chatham Dockyard from the north".

On the roof of the fort, accessed from the ground level by two spiral staircases, there was an observation post protected by a banquette (an elevated step to facilitate rifle fire against attackers at close range).

Their interiors were also partly infilled with rubble, as was the defensive ditch in front of the casemates, providing additional protection against enemy fire as it helped to camouflage the structure.

Two light quick-firing guns with ammunition lockers were installed on the fort's roof to guard against small, fast-moving threats such as torpedo boats.

[1] Slough Fort is well-preserved and still substantially intact, though it has suffered some damage over the years (notably the loss of the outermost gun position of the left wing battery) and is surrounded on three sides by the caravans of the Allhallows Leisure Park.

English Heritage designated it as a Grade II* listed building in 2009 in recognition of its unusually good state of preservation and for the way that it embodies "both technological developments in armaments and coastal defence during the late-C19 to early-C20 as well as the changing views of the perceived defensive need on the north Kent coast at this time.

The wing batteries have been cleared of thousands of tons of accumulated debris, a walking trail linking them has been laid out and interpretative signs have been installed for the benefit of visitors to show how the fort once appeared and how it functioned when it was in use.