A historic sea fort near the entrance of Portsmouth harbour is to be sold at auction next month.
Spitbank Fort, which once defended the Solent, will go under the hammer at the Rose Bowl stadium in Southampton.
Described by its owners as the ultimate venue, the property has a guide price of between £600,000 and £700,000.
The 50-room property is being sold by entrepreneurs Neale Brickwood, Paul Holland and Ian Price who bought it for an undisclosed sum in July 2005.
The 148-year-old fort, which also has permission for a casino, is being sold through auctioneers Clive Emson on 6 November.
The current owners have been holding functions at the venue, which is licensed for up to 500 visitors.
The fort was built during the Napoleonic wars
"Spitbank Fort reportedly cost nearly £120,000 to construct between 1861 and 1878 and was one of Palmerston's 'Follies', a series of land and sea-based forts designed to repel French warships," said auctioneer Rob Marchant.
"The fort was the Trident missile of its time, a crucial part of the defence of the realm."
During World War II it was installed with anti-aircraft guns to defend nearby Portsmouth harbour against Nazi attack.
It was decommissioned in the 1960s and turned into a museum.
The walls are 15ft (4.5m) thick at basement level, with 35ft (10.6m) thick sea foundations and a 8ft (2.3m) thick concrete roof.
Mr Brickwood said: "It's not every day you get to own an armoured fortress in the middle of one of the busiest waterways in the world and with some of the best vistas in the kingdom.
"It is being sold with everything necessary to run the fort as a business, from the rigid inflatable boat to the cups and saucers, knives and forks."
Spitbank is one of a series of forts in the Solent. No Mans Land Fort has been converted into residential accommodation, while Horse Sand Fort has remained derelict for about half a century.
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