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

article-imageFort Louvois at high tide (photograph by Lionel Maraval)

Built: 1691-1694
Purpose: 
Protect the Château d'Oléron, didn't see any action until WWII
Current Use: 
Oyster farming museum

article-imageFort Louvois at low tide (photograph by Roromain69/Wikimedia)

Shivering Sands Army Fort
Thames Estuary, England

article-imagephotograph by Hywel Williams

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

article-imagephotograph by Hywel Williams

Murud-Janjira
Maharashtra, India

article-imagephotograph by Sagar Jadhav

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

article-imagephotograph by Himanshu Sarpotdar

Fort Boyard
Pertuis d'Antioche Straits, France

article-imagephotograph by Mpkossen/Wikimedia

Built: 1801-1857
Purpose: 
Protect a navy arsenal
Current Use: 
French game show set

article-image
The fort in the distance from the beach (photograph by Frédérique Voisin-Demery) 

Fort Jefferson
Dry Tortugas, Florida, United States

article-imageArriving by boat to Fort Jefferson (photograph by Matt Kieffer)

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

article-imageFort Jefferson moat wall (photograph by Matt Kieffer) 

Fort Alexander
Saint Petersburg, Russia

article-image
photograph by Serh113/Wikimedia

Built: 1838-1845
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

article-imagephotograph by Florstein/Wikimedia 

No Man's Land Fort
Isle of Wight, England 

article-imagephotograph by Colin Babb

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 

article-imagephotograph by Andrew/Flickr user

Nab Tower
Isle of Wight, England

article-imagephotograph by JOG/Flickr user

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

article-imagephotograph by JOG/Flickr user 

Fort Carroll
Baltimore, Maryland, United States

article-imagephotograph by Jon Dawson

Built: 1848
Purpose:
Protect Baltimore
Current Use: 
Abandoned

article-imagephotograph by Cristina/Flickr user

Spitbank Fort
Portsmouth, England

article-imagephotograph by Amanda Retreats

Built: 1861-1878
Purpose: 
Protect Portsmouth
Current Use: 
Luxury spa and hotel

article-image via Wikimedia

Fort Brehon
St Peter Port, Guernsey

article-imagephotograph by Unukorno/Wikimedia

Built: 1854-1856
Purpose: 
Protection from the British
Current Use: 
Was an anti-aircraft site in WWII, now is ruins and a Common Tern breeding ground

article-imagephotograph by Man vyi/Wikimedia

Fort Denison
Sydney Harbour, Australia

article-imagephotograph by icameronbps/Flickr user

Built: 1841-1857
Purpose: 
Protection from foreigners after the appearance of American warships
Current Use: 
Tourist site

article-imagephotograph by Mike Brand

Trekroner Fort
Copenhagen, Denmark

article-image
photograph by Thue C. Leibrandt

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

article-image
photograph by denkrahm/Wikimedia 

St Helens Fort
Isle of Wight, England

article-imageView from shore (photograph by Andrew/Flickr user)

Built: 1867-1880
Purpose: 
Protect Portsmouth
Current Use: 
Privately owned, although people walk there during the few hours of low tide in the summer

article-image
The St. Helens Fort walk during low tide (photograph by Richard Heaven) 

Places in this article

Fort Louvois

Bourcefranc-le-Chapus, France

Murud Janjira fort

Raigad, India

No Man's Land Luxury Sea Fort

Nettlestone, England

Maunsell Army Sea Forts

Canterbury, England

Fort Alexander

Saint Petersburg, Russia

Fort Boyard

Île-d'Aix, France

Dry Tortugas

Key West, Florida
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