The Impenetrable Island Isolation of Sea Forts - Atlas Obscura
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The Impenetrable Island Isolation of Sea Forts

14 amazing sites where waves meet walls.

Fort Louvois at high tide.
Fort Louvois at high tide. Lionel Maraval/Flickr/CC BY-SA 2.0

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

Fort Louvois at low tide.
Fort Louvois at low tide. Henri-Jean Siperius/Flickr/CC BY-SA 2.0

Built: 1691–94
Protect the Château d’Oléron, didn’t see any action until WWII
Current Use:
Oyster farming museum

Maunsell Army Sea Forts

Thames Estuary, England

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

Built: 1943
Anti-aircraft defense
Current Use:
After a post-military career in pirate radio and employment as search lights, they were abandoned.


Raigad, India

Murud-Janjira Ishan Manjrekar/Flickr/CC BY-SA 2.0

Built: 15th Century
Purpose: Protection from pirates
Current Use:

Fort Boyard

Saint-Georges-D’oléron, France

Fort Boyard.
Fort Boyard. Armel/Flickr/CC BY-SA 2.0

Built: 1801–57
Protect a navy arsenal
Current Use:
French game show set

Fort Jefferson

Dry Tortugas, Florida, United States

Dry Tortugas.
Dry Tortugas. Matt Kieffer/Flickr/CC BY-SA 2.0

Built: 1824-never finished
Stop pirates
Current Use:
National Park

Fort Alexander

Saint Petersburg, Russia

Fort Alexander.
Fort Alexander. Andrew Shiva/Wikipedia/CC BY-SA 4.0

Built: 1838–45
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

No Man's Land Fort.
No Man’s Land Fort. David Jones/Flickr/ CC BY-SA 2.0

Built: 1867-1880
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

Nab Tower.
Nab Tower. 27col/Wikimedia Commons/CC BY-SA 4.0

Built: WWI
Stop submarines
Current Use:
Lighthouse, and sailboat race destination

Fort Carroll

Baltimore, Maryland, United States

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

Built: 1848
Protect Baltimore
Current Use:

Spitbank Fort

Portsmouth, England

Spitbank Fort.
Spitbank Fort. mdbgallery/Flickr/CC BY-SA 2.0

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

Brehon Tower

St. Peter Port, Guernsey

Brehon Tower.
Brehon Tower. Unukorno/Wikimedia Commons/CC BY-SA 2.0

Built: 1854–56
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

Fort Denison.
Fort Denison. russellstreet/Flickr/CC BY-SA 2.0

Built: 1841–57
Protection from foreigners after the appearance of American warships
Current Use:
Tourist site

Trekroner Fort

Copenhagen, Denmark

Trekoner Fort.
Trekoner Fort. Thue C. Leibrandt/Wikimedia Commons/CC BY-SA 3.0

Built: 18th Century
Protect Copenhagen
Current Use:
Tourist site

St. Helens Fort

Isle of Wight, England

St. Helens Fort.
St. Helens Fort. Editor5807/Wikimedia Commons/CC BY-SA 2.0

Built: 1867–80
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.