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.

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

Thames Estuary, England

Red Sands Forts of the Maunsell Army Sea Forts. Russss/Wikimedia Commons/CC BY-SA 3.0

Built: 1943

Purpose: Anti-aircraft defense

Current Use: After a post-military career in pirate radio and employment as search lights, they were abandoned.

Raigad, India

Built: 15th Century

Purpose: Protection from pirates

Current Use: Abandoned

Saint-Georges-D’oléron, France

Built: 1801–57

Purpose: Protect a navy arsenal

Current Use: French game show set

Dry Tortugas, Florida, United States

Built: 1824-never finished

Purpose: Stop pirates

Current Use: National Park

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

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



Baltimore, Maryland, United States

Fort Carroll. Larry Myhre/Flickr/CC BY-SA 2.0

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.