Great guest post from Weather.com of all places:
1. Solent Forts (No Man’s Fort and Spitbank Fort), England
Built in the 1870s and 1880s to repel the French navy, the Solent Forts, located off the south coast of England have stood immovable for centuries.
Today, they are luxurious spa hotels and retreats. The historic sea-granite Spitbank Fort features a circular plunge pool and sauna in its former rooftop gun emplacement, and its ammunition store has been converted into a wine-tasting room.
It has eight bedroom suites offering sea views, bar, sunbathing decks, restaurant, and even fire pits, according to the Daily Mail. No Man’s Fort has also been converted into a luxurious getaway, and boasts four floors and 75,000 square feet of entertainment, recreation, and dining, according to Yahoo Travel.
2. Fort Jefferson, Dry Tortugas National Park, Florida
Fort Jefferson was built to protect one of the most strategic deepwater anchorages in North America, serving as an important “advance post” for ships patrolling the Gulf of Mexico and the Straits of Florida.
Nearly thirty years in the making, it was never finished nor fully armed. Abandoned by the Army in 1874, the fort was later used as a prison and as a coaling station for warships, according to the National Park Service.
Today, it’s part of the 100-square mile Dry Tortugas National Park, home to incredible coral reefs and marine life, and the vast assortment of bird life that frequent the area. It’s accessible only by boat or seaplane.
3. Maunsell Sea Forts, England
Named after their designer, Guy Maunsell, the Maunsell Forts are armed towers, operated as army and navy forts, built in the Thames and Mersey estuaries in England during the Second World War, when Great Britain faced serious attacks from the German air force.
Though collectively call the Maunsell forts, there were two designs with different purposes—army forts and as naval forts. The army forts were connected through a walkway, while the naval forts have two cylindrical towers that are united by a gun platform above. They were abandoned in 1958, according to MaunsellSeaForts.com.
They have now become tourist attractions, primarily due to their bizarre shape. Some boat tours to the forts are available, although exploring the forts may be dangerous because the structures are in an advanced state of decay.
4. Fort Boyard, France
Located on the west coast of France, Fort Boyard was built to protect the ports of Aix Island and Rochefort. Construction of the fort started in 1801 and it was completed in 1857.
It served as a prison at the end of the 19th century, before being abandoned by the army around 1910. It has been featured in movies and is the location of popular adventure game show, also called “Fort Boyard.” Although the fort is not open to the public, several boat tours provide great views from the sea.
5. Fort Denison, Australia
Fort Denison was a former penal site and defensive facility occupying a small island located in Sydney Harbour, New South Wales, Australia.
The fortress features a distinctive Martello tower, the only one ever built in Australia and the last one every constructedin the British Empire. In its 150 years, Fort Denison has been used as a defense structure, fishing spot, navigational guide, tide gauge station, weather station, and time marker.
It is now part of Sydney Harbour National Park and is also used as a nature reserve, tourist facility, and as an event space. It also houses a restaurant and museum. Tourists can book guided tours of the fort through the Australian National Parks and Wildlife Service.
6. Fort Alexander, Russia
Located on an artificial island in the Gulf of Finland near St. Petersburg and Kronstadt, Russia’s Fort Alexander was a naval fortress built to guard the Baltic waterway. It never participated in any military action and later housed a research laboratory on plague and other bacterial diseases.
The fort was abandoned in the 1980s and became known as a site for illegal rave parties and urban exploration favorite, according to Atlas Obscura. Today, tourists can take boat tours to the fort.
7. Fort Sumter, South Carolina
A sea fort located in Charleston, South Carolina, Fort Sumter is notable for two battles of the American Civil War. The Battle of Fort Sumter on April 12–14, 1861 was the start the American Civil War, when Confederate artillery opened fire on the fort.
Fort Sumter is now a National Monument, with a museum, which focuses on the activities at the fort, including its construction and role during the Civil War, and visitor center, which features exhibits about the disagreements between the North and South that led to the incidents at Fort Sumter. Boat tours to the fort are available.
8. Murud-Janjira Fort, India
Murud-Janjira Fort is located just off the coastal village of Murud, in Maharashtra, India. It is considered one of the strongest marine forts in India and is said to be the only unconquered one along India’s western coast.
Though it hasn’t been maintained, the fort, which is known for its three massive cannons, has become a popular tourist attraction and is accessible via ferry.
9. Flakfortet, Denmark
Flakfortet (which means “sand-shoal fortress”) is a sea fort on an artificial island located in the middle of the Sound between Denmark and Sweden. It was created to defend the entrance to Copenhagen’s harbor but its military functions ended in 1968.
Today, the fort is a venue for concerts and events (treasure hunts are held there). It also houses an overnight hotel, mini-golf course and a restaurant that offers amazing views. Excursions to the fort via ferry are offered during the summer season.
10. Fort Pampus, Netherlands
Fort Pampus is a late 19th-century sea fort located in the IJmeer near Amsterdam, Netherlands. Part of the Defence Line of Amsterdam, a UNESCO World Heritage Site, Pampus was built to protect the entrance to IJ Bay and the harbour of Amsterdam.
In 2007, the fort was partially restored. Today, Fort Pampus is open to visitors during the months of April to October, and accessible via ferry.