The site that Shoreham fort sits is a shingle spit, 1.6 hect (4 acres) on the west bank, where the river Adur exits to the sea. The spit being separated from the mainland for nearly 2 miles (3.2km) by the coarse of the river Adur, until the river then makes it coarse inland, just near Shoreham airport.
The fort was designed to position six muzzle-loaded guns with rifled barrels but the survey drawing records only five mounted at the time. On completion of the fort it was armed with six 68-pounders later replaced by 64-pounders on emplacements I and VI and 32-pounders on emplacements II, III, IV and V, the emplacements stood about 15 ft (4.6m) above sea level, so the visibility over the harbour entrance and neighbouring beaches would be increased, increasing the defence of the fort. The underground magazines were each designed to take 126 barrels of gunpowder and the water tanks carried 11,578 gallons.
The fort was built to accommodate two Officers, one Master Gunner and 35 NCO's and Privates all housed in the barracks.
The ground plan was in the shape of a lunette, or a rectangular half-moon, similar to the fort at Littlehampton, with earthen ramparts on which the guns were mounted, and at the rear was a defensible barrack block. The fort was surrounded at the front and sides by a ditch which carried a Carnot wall along the bottom, this wall would have been about 12 ft (3.7m) high. Shoreham is the earliest example of a fort with a Carnot wall still reasonably intact in the UK .
At the three corners are the covered bastions, or Caponiers/caponierres, which can be entered from the inside of the fort, allowing defenders to fire along the outside of the Carnot wall whilst still being under cover. These represent a development from the open bastions built a few years earlier at Littlehampton, it was felt that if the men were defended by a roof, it would make an invasion impossible to succeed, as all men were then defended at all angles. The Caponiers were identical to that built to protect the main entrance to Hurst Castle , which is one of the huge granite casemate fortifications built to protect the needles entrance to Southampton Water.
Buried beneath the two ends of the ramparts were the two magazines, one of which is now incorporated into a coastguard tower.
These comprised stores and shifting rooms where the shells and cartridges were loaded. Piles of iron shot were placed by each gun and shell recesses or expense magazines, where small supplies of ammunition were maintained lay adjacent. There were no hoists, and shells were carried to the guns by hand, probably by making use either of the steps alongside and above the expense magazines or the ramp.
The guns, mounted on the gun platform or terreplein, fired over a low protective wall. The wooden gun carriage recoiled up an inclined plane on a traversing wooden platform carried on iron rails. Each gun was manned by at least seven men and was manoeuvred using wedges, levels and block and tackle. the Gunnery Officers could supervise operations from the steps placed between the gun emplacements, which could also be mounted infantry to fire muskets at enemy troops approaching on foot from the beach.
The Barrack Block accommodated officers and men and, with its rifle slits, formed part of the fort defences. A central area served as a parade ground and beneath this were two tanks which, if needed, could supply the fort with water in time of attack. The water tanks approximately carried 11,578 gallons of water.
Soon after the fort was completed it was proposed that the fort should be remodeled after criticism in parliament. Apart from the difficulties traversing the guns because there was insufficient room behind the parapet, it was also pointed out that if round shot hit the beach in front of the fort it would have the affect of grape shot over the men exposed when working the guns. This criticism seems to have been justified and lead to a new inquiry in about 1859 and three additional fortification expansion works were generously proposed.
1, A battery of five guns immediately opposite the harbour entrance, where the lighthouse stands today
2, A strong casemated work of 12 guns and accommodation for 200 men. This was to be built on the west side of the harbour entrance, although no one is sure whether it would be where the original fort was built or whether they would keep the original and build near it
3, A new work for a 6 gun battery on the neck of the eastern promontory between Aldrington Basin and the sea. This battery would stand near the site of the power station and was intended to discourage ships lying offshore from bombarding craft in the harbour
Although this work never happened the fort was still manned by the volunteers until at least 1896, so the fort was manned for the short period of roughly 36 years
In the Second World War a battery of six-inch guns was erected on the fort, but these have since been removed and only part of the footings survive, It may have been at this stage, or even earlier, when the gun emplacements I, III and VI were modified by the lowering of the walls over which the guns fired. There was also a searchlight tower constructed on the western side of the fort, which still survives today. The Barrack Block was variously used as a film studio and private dwelling before being demolished in 1959.