The monument includes a circular, casemated battery, set within an unrevetted ditch and outer glacis, with associated groynes, jetty and the remains of later, World War II structures. Fort Darnet is one of a pair of batteries, its twin being Hoo Fort, constructed on low islands on opposite sides of the Medway channel. Hoo Fort is the subject of a separate scheduling. They were built during the 1860s on the recommendation of the Royal Commission on Defence, and were intended to provide an inner line of defence to protect the approaches to the naval dockyard at Chatham. An additional safeguard, in the form of a minefield, laid across the channel between the forts, was to be employed in the event of war. The major fortifications at Grain and Sheerness supplied the outer line of defence at the mouth of the Medway. These fortifications are also the subject of separate schedulings. The substantial, two-tiered battery, measures around 56m in diameter externally, and stands to its original height of about 10m. The brick-built structure is faced in granite ashlar, with lower courses dressed in Kentish ragstone. The upper level, smaller in diameter than the tier below, protrudes above the lip of the encircling ditch, beyond which, a sloping bank, or glacis, extends for a distance of up to 50m. Traces of associated timber structures can be seen along the foreshore to the north and west of the fort, including the remains of groynes, and the jetty onto which supplies and ammunition were unloaded. The fort was originally entered at ground floor level, through a passage on its southern side, and was approached from he jetty by way of a curving footpath, crossing the southern slope of the glacis. The entrance passage is no longer accessible, due to flooding of the ditch and lower level of the fort, although the upper tier is reached by way of a modern footbridge from the roof of the brick, vaulted coalstore situated within the ditch. The lower tier contains the magazine and accommodation casemates, arranged in concentric rings around a solid concrete drum at the centre of the fort. The outer ring of magazine chambers represent the shell and cartridge stores, which were entered from the magazine passage in front. Lift shafts also rise from the passage enabling the rapid deployment of ammunition to the gun floors above. A sophisticated lighting system formed part of the safety features of the magazine and consisted of a lantern window above the door to each chamber, and separated from it by a pane of glass. The lamps were carried above the magazine passage on horizontal, overhead rails, contained within zinc conduits, and were served by a ring of lamp chambers, accessed from the barrack rooms beyond. In turn, the barrack rooms were entered from an open corridor, or light well, which surrounds the central drum. Steps led up from the corridor to the top of the drum, which provided a small, open parade at the centre of the gun level. The gun level contains an outer ring of 11 interconnecting, vaulted casemates, arranged around the parade and reached by narrow bridges across the light well. The casemates were designed to accommodate eight 9in, and three 7in rifled muzzle-loaders mounted on traversing carriages. The casemates retain many of their original features, including the iron shields inserted into the embrasures for the protection of the gunners. Rope mantlets were also hung behind the shields to reduce casualties from masonry splinters in the event of enemy fire, and some of their suspension bars and rings survive. The chamber behind the gun room was intended to provide wartime accommodation for the gunners, and was enclosed at the rear by a glazed screen, designed to be removed before the guns were fired. The screens have now been lost, along with the glazed verandah, originally constructed around the parade perimeter. The verandah formed part of the rainwater collection system for the fort, and was supported on hollow cast iron columns, through which rainwater was fed to a cistern beneath the parade. Reuse of the fort during World War II is represented by a minewatching post, situated on the roof, overlooking the Medway to the north and west, and an observation post constructed within the embrasure of a southern casemate, with views towards South Yantlet Creek. All modern fixtures and fittings, such as the temporary footbridge across the ditch and the remains of the modern jetty on the south western edge of the glacis are excluded from the scheduling, although the ground beneath all of these features, or the structures to which they are attached, is included. Also excluded are the concrete hulks from World War II, laid up along the south western side of the island during the late 20th century, where they fall within the area of protection.