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Murud-Janjira, India -

One of the world's most spectacular sea forts, Murud-Janjira is situated on an island just off the coastal town of Murud, in the Raigad district of Maharashtra, India. Built in the 15th century, Janjira proved virtually impregnable and is considered one of the strongest coastal forts in the country.

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Fort Jefferson, USA -

The largest brick masonry structure in the Americas, Fort Jefferson in the Dry Tortugas National Park of Florida is one of the country's most revered historical landmarks. Built in 1847 to protect a strategic deep water anchorage, the fort went on to play a pivotal defensive role during the American Civil War.

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Red Sands Towers, England -

The Red Sands Towers are a collection of Second World War-era iron structures in the Thames Estuary. They were designed for the army to defend London from the German Luftwaffe and V1 and V2 rockets.

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São Marcelo Fort, Brazil -

São Marcelo Fort sits in the Baía de Todos os Santos and faces the city of Salvador in Bahia, Brazil. A local landmark, the fort was built in the early 17th century, and played a decisive role in the defense of the city during the Dutch rule of Northeast Brazil from 1630 to 1654.

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Fort Boyard, France -

The solid-looking, stadium-shaped Fort Boyard was completed in 1857. It stands in the Pertuis d'Antioche straits, on the west coast of France, and was built to protect the arsenal of Rochefort from British Royal Navy incursions. Today, it serves as the location for the popular French game show 'Fort Boyard' (1990–present).

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Fort Pampus, Netherlands -

Constructed in the late 19th century, Fort Pampus sits on an artificial island in the IJmeer near Amsterdam. It's one of 42 forts built as part of the Defence Line of Amsterdam. Today, the entire ring of fortifications is a UNESCO World Heritage Site.

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Kızkalesi, Turkey -

Kızkalesi, which means "Maiden's Castle" in Turkish, is a picturesque island castle set in the Mediterranean Sea in Turkey's Mersin province. The castle's foundations date back to at least the 13th century.

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Fort Sumter, USA -

Another of America's most historic sea defense facilities, Fort Sumter straddles an artificial island near Charleston, South Carolina. The fort was still incomplete in 1861 when the Battle of Fort Sumter occurred, sparking the American Civil War. It's now a popular visitor attraction.

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Horse Sand Fort, England -

One of four structures built in the 19th century as so-called Palmerston Forts, Horse Sand Fort in the Solent off the south coast of England is the largest of the quartet. The Palmerston Forts, named after Lord Palmerston, were built in response to the perceived threat of a French invasion.

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Fort Alexander, Russia -

Fort Alexander is a naval fortress on an artificial island in the Gulf of Finland near St. Petersburg and Kronstadt. The bean-shaped stronghold is also known as "Plague Fort" after scientists chose it as a location to conduct research into Yersinia pestis, the bacterium that causes the plague, during the late 1800s. The lab shut down in 1917.

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Fort Louvois, France -

One of France's best-known sea defense structures, identified by its horseshoe-shaped battery, Fort Louvois was built between 1691 and 1694, during the reign of Louis XIV. It stands on the Chapus islet near the town of Bourcefranc-le-Chapus.

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Flakfortet, Denmark -

Located in the Øresund just a short boat trip from central Copenhagen, Flakfortet is one of three artificial islands guarding the entrance to Copenhagen Harbor. The fort itself, completed in 1914, today serves as a cultural center.

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Trekroner Fort, Denmark -

Trekroner Fort was also built to defend the entrance to the Danish capital, and complements Flakfortet and Middelgrundsfortet. The fort served as an important part of the Danish line of defense during the Battle of Copenhagen in 1801, and also saw action during the British attack on Copenhagen in 1807.

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Fort Carroll, USA -

Named for Charles Carroll, a signatory of the Declaration of Independence, Fort Carroll was completed in 1850 to protect the approach to Baltimore from the Patapsco River. The island and its characteristic hexagonal sea fort is now abandoned.

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Fort of São Lourenço do Bugio, Portugal -

Commonly referred to simply as Torre do Bugio, the Fort of São Lourenço was built between 1590 and 1657 to protect the approach to the Tagus river and Lisbon. The fort was garrisoned until at least 1911 before it was repurposed as a lighthouse.

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Fort St. Angelo, Malta -

Located at the center of the Grand Harbor in Birgu, Malta, Fort St. Angelo is an impressive 16th-century bastioned fortress best known for its role as the headquarters of the Order of St. John during the Great Siege of Malta of 1565.

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Bourtzi Nafplio, Greece -

Anchored in the middle of Nafplio Harbor in Greece is Bourtzi Castle. Dating back to 1471, it's Venetian in origin and is now a fully refurbished and hugely popular tourist attraction.

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Fort Denison, Australia -

One of Sydney Harbor's more unusual landmarks, Fort Denison was completed in 1857 to protect the harbor from attack by foreign vessels. The island, however, was originally the site of a penal facility to house convicts.

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Fort de Bertheaume, France -

Fort de Bertheaume's foundations date back to the 14th century. The garrison was strengthened in the 15th century to monitor the Goulet de Brest, the straits of Brest, and to repel potential English attack.

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Fort Monroe, USA -

Fort Monroe occupies the southern tip of the Virginia Peninsula, within the boundaries of Old Point Comfort. A former military installation, Fort Monroe was completed in 1834. Although Virginia became part of the Confederate States of America, Fort Monroe remained in Union hands throughout the American Civil War. Incidentally, the fort is noted as the location of the arrival of the first Africans to English-speaking North America.

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Fort Mahon, France -

Designed by French military architect Vauban, considered the greatest engineer of his time, Fort Mahon was built on the orders of Louis XIV to defend the port of Ambleteuse at the mouth of the Slack river.

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Suomenlinna, Finland -

Suomenlinna comprises eight islands, of which six have been fortified. The islands lie about 2.5 mi (4 km) southeast of the city center of Helsinki. The design of the bastion fortifications were strongly influenced by the work of the aforementioned Vauban, and it's for this that UNESCO has listed Suomenlinna as a World Heritage Site.

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Humber Forts, England -

The Humber Forts—Bull Sand Fort and Haile Sand Fort—are two large fortifications in the mouth of the Humber Estuary in northern England. Originally planned for service in the First World War, the forts instead saw action during the Second World War.

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Fort Gorges, USA -

Perched on Hog Island Ledge in Casco Bay, Maine, Fort Gorges was built for purpose from 1858 to 1864, but no troops were stationed there, nor battles fought. The fort was last used by the Army during the Second World War, when it was used to store submarine mines.

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Peñón de Vélez de la Gomera, Spain -

Peñón de Vélez de la Gomera is a Spanish exclave and rocky tidal island, which is crowned by a small garrison manned by a modest complement of military personnel. The island, located in the western Mediterranean Sea, is connected to Morocco by a sandy isthmus, making it one of the shortest international borders in the world.

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Fort Drum, Philippines -

Fort Drum was built by the United States in 1909 as a heavily fortified island in the mouth of Manila Bay in the Philippines. Constructed of reinforced concrete and shaped like a battleship, Drum saw action during the Battle of Corregidor (1942) and throughout the Philippines campaign of 1944–1945.

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Fort Nathan Hale, USA -

Fort Nathan Hale's modest dimensions belies its place in American military history as one of the most important defensive facilities of the 18th and 19th centuries. Named after Nathan Hale, a Patriot who spied for the Continental Army during the American Revolutionary War and who today is considered a national hero, the fort saw action in that conflict and the War of 1812, protecting Connecticut from British attack.

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Fort Clinch, USA -

Fort Clinch is actually a coastal defense installation but is included on this list for the role it played during the American Civil War. Confederate and Union armies both occupied the stronghold during the conflict, with Federal forces using Clinch as the base of Union operations in the area throughout the period. The US Army also garrisoned it during the short Spanish–American War.

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Stack Rock Fort, Wales -

It was Thomas Cromwell who first proposed a fortification at Stack Rock, a small island in the Milford Haven Waterway in Pembrokeshire. Over 300 years later, in 1852, a garrison was finally completed to monitor the waterway. Stack Rock Fort is one of the very few Victorian sea forts in the United Kingdom still surviving relatively intact.  

Sources: (Live Science) (National Park Service)  

See also: Europe's prettiest hilltop towns and villages

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