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. Hoo Fort is one of a pair of batteries, its twin being Fort Darnet, constructed on low islands on opposite sides of the Medway channel. Fort Darnet 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 river. These 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, rotrudes above the lip of the encircling ditch, beyond which, a sloping bank, or glacis, extends for a distance of up to 30m. Traces of associated timber structures can be seen along the foreshore to the north east and south of the fort, including the remains of groynes, and the jetty, onto which supplies and ammunition were unloaded.The fort is entered at ground floor level, through a passage on its north western side, and was approached from the jetty by way of a curving footpath, crossing the western slope of the glacis. The entrance passage is flanked internally by two small chambers, which retain parts of the mechanism for raising a section of the passage floor, designed to act as a drawbridge in the event of attack. The passage also provides access to the magazine and accommodation casemates of the lower tier, 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, and are entered from the magazine passage in front. Lift shafts 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, set into the wall above the door to each chamber, and separated from the chamber by a pane of glass. The lamps were carried across 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 are entered from an open corridor, or light well, which surrounds the central drum. Steps lead up from the corridor onto the top of the drum, which provided a small, open parade at 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 eleven 9in 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 south east. Additional features beyond the area of the monument include traces of a low circular earthwork on the northern tip of the island, considered to represent an initial attempt to construct the fort, and now partly destroyed and submerged by the river.