Spitbank Fort is now part of the seascape of the Solent - a great granite bastion between Portsmouth and the Isle of Wight.
It was initially designed to defend Portsmouth's naval dockyard from French invasion - today it stands as a testament to Victorian engineering.
Spitbank is one of four sea forts built in the Solent - with St Helens Fort, Horse Sand Fort and No Mans Land Fort.
It islocated a mile (1.6km) off the coast of Portsmouth.
Inside Spitbank Fort
The story of Spitback Fort goes back to 1851 when Napoleon III became Emperor of France, sparking widespread fear in Britain that a French invasion was likely.
Prime Minister, Lord Henry Palmerston, initially commissioned five forts to run along the Solent's eastern approaches to defend the Royal Navy fleet at anchor in Portsmouth harbour.
Four remain today as one was abandoned during construction.
The granite and iron sea forts were built with cutting edge Victorian engineering technology.
However they, along with other defensive forts along the south coast, were later dubbed "Palmerston's Follies" as the French threat never materialised.
The construction of Spitbank began in 1861, but stopped due to the political wranglings.
Spitbank Fort was built to defend Portsmouth from naval attack
Work resumed in 1864, with the first stone being laid in March 1867 and completion taking place in June 1878.
The total cost of the fort was £117,964, plus the ironwork.
Nicholas Hall - keeper of artillery, Royal Armories, Fort Nelson says:
"The Victorians seem to have been supremely self-confident when it came to large engineering projects.
"They had to use divers, to place the first stones which weighed several tonnes and were beautifully cut and fitted. The forts are amazing constructions."
At its peak, a garrison of nearly 30 maintained the fort which was run like a ship.
Life below "deck" being remarkably similar to that on the sea - with soldiers sleeping in triple-banked hammocks.
A labyrinth of tunnels within the fort
However for the servicemen who "crewed" the forts, it was an isolating experience.
"It must have been very unpleasant, there isn't actually much room, it was cold, damp and miserable and they had go on a gantry platform to go to the toilet. It must have been grim," explains Mr Hall.
Spitbank fort was designed to defend Portsmouth Harbour from French invasion, however when that threat was never realised, the sea forts, along with land forts like nearby Fort Nelson and Fort Widley, were branded as "Palmerston's Follies."
However Mr Hall sees Spitbank Fort as being militarily significant.
"In a sense the term applies less to the sea forts as they do stand in the approaches and could, and were, used as defences for much longer than the land forts," he said.
"Militarily speaking, they were an important part of the defence of the dockyard up to and including World War II.
Gun emplacements on the forts were part of Portsmouth's coastal artillery defences which were controlled from Southsea Castle during World War II.
The fort commands views over the Solent
Its military role ended in 1952. Since then it has had a succession of private owners, the latest buying the fort for more than £1m, it remains a visible symbol of Portsmouth's maritime and military heritage.
"Spitbank is a most evocative place to visit. The forts can look quite forbidding from the outside and are fascinating places - there is nothing else like them.
"It is a monument to Victorian engineering and to the need to defend the premier naval dockyard. It's visual impact remains very striking to anyone walking on the shore or sailing out on the water," said Mr Hall.
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