The Impenetrable Island Isolation of Sea Forts

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

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

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

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

Built: 1848
Purpose: Protect Baltimore
Current Use: Abandoned

Spitbank Fort

Portsmouth, England

Built: 1861–78
Purpose: Protect Portsmouth
Current Use: Luxury spa and hotel

Brehon Tower

St. Peter Port, Guernsey

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

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

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.