The Impenetrable Island Isolation of Sea Forts
14 amazing sites where waves meet walls.
These islands are fortresses. Whether built up to consume tiny landforms or constructed on manmade foundations, sea forts are outposts of military might now left stranded in the seas. With accessibility reliant on the tides or boats, some sea forts are abandoned after they become obsolete, others take on second lives as hotels, bird nesting sites, or even game show sets. Here are some strongholds for if you ever want to get away for some impenetrable solitude.
Fort Louvois
Bourcefranc-le-Chapus, France

Built: 1691–94
Purpose: Protect the Château d’Oléron, didn’t see any action until WWII
Current Use: Oyster farming museum
Maunsell Army Sea Forts
Thames Estuary, England

Built: 1943
Purpose: Anti-aircraft defense
Current Use: After a post-military career in pirate radio and employment as search lights, they were abandoned.
Murud-Janjira
Raigad, India
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Built: 15th Century
Purpose: Protection from pirates
Current Use: Abandoned
Fort Boyard
Saint-Georges-D’oléron, France

Built: 1801–57
Purpose: Protect a navy arsenal
Current Use: French game show set
Fort Jefferson
Dry Tortugas, Florida, United States

Built: 1824-never finished
Purpose: Stop pirates
Current Use: National Park
Fort Alexander
Saint Petersburg, Russia
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Built: 1838–45
Purpose: Fortify the Gulf of Finland
Current Use: After serving as a research center for the plague and rave party site in the 1990s, it is now open to tours
No Man’s Land
Isle of Wight, England
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Built: 1867-1880
Purpose: Protect Portsmouth
Current Use: Served as a high end private hotel, closed for health reasons, now under new owners who intend to reopen it as a hotel
Nab Tower
Isle of Wight, England

Built: WWI
Purpose: Stop submarines
Current Use: Lighthouse, and sailboat race destination
Fort Carroll
Baltimore, Maryland, United States
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Built: 1848
Purpose: Protect Baltimore
Current Use: Abandoned
Spitbank Fort
Portsmouth, England
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Built: 1861–78
Purpose: Protect Portsmouth
Current Use: Luxury spa and hotel
Brehon Tower
St. Peter Port, Guernsey
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Built: 1854–56
Purpose: Protection from the British
Current Use: Was an anti-aircraft site in WWII, now is ruins and a Common Tern breeding ground
Fort Denison
Sydney, Australia
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Built: 1841–57
Purpose: Protection from foreigners after the appearance of American warships
Current Use: Tourist site
Trekroner Fort
Copenhagen, Denmark

Built: 18th Century
Purpose: Protect Copenhagen
Current Use: Tourist site
St. Helens Fort
Isle of Wight, England
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Built: 1867–80
Purpose: Protect Portsmouth
Current Use: Privately owned, although people walk there during the few hours of low tide in the summer
This article was updated on November 27, 2018, with new images and minor edits.
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