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Bourtange Fort, Netherlands -

Bourtange Fort was originally built during the Eighty Years' War between 1568 and 1648. This amazing star-shaped fortress eventually became a village.

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Fortress of Suomenlinna, Finland -

The fortress was built in the second half of the 18th century by Sweden. Located on six islands at the entrance of Helsinki’s harbor, this amazing structure features over 200 buildings.

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Windsor Castle, England -

Windsor Castle was built by William the Conqueror in 1070 in a strategic location along the River Thames. Queen Elizabeth II uses the palace fairly often, and Windsor Castle remains the oldest and largest inhabited castle in the world.

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Alcázar of Toledo, Spain -

Dating back to the Roman Empire, Alcázar of Toledo suffered a number of changes throughout the following centuries. Sitting at the top of the city of Toledo, it played a crucial role in the country's history.

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Masada, Israel -

Herod the Great built Masada around 31 BCE on a plateau on the Judean Desert, overlooking the Dead Sea. It had two palaces and a church at one point.

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Fortress of the Orsini, Italy -

The Fortress of the Orsini was built in the 14th century. It features five underground levels and is equipped with cisterns to collect rainwater. It also has a windmill.

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Area 51, Nevada, USA -

The mystery and secrecy surrounding Area 51 is what makes it so special. The CIA didn't publicly acknowledge the existence of the United States Air Force base until 2013.

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Rumeli Castle, Turkey -

Also known as Rumelihisarı or Boğazkesen Castle, this fortress was built in Istanbul circa 1451. It was originally built in preparation for a siege of Constantinople (Roman Empire).

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Saxon Bastille, Germany -

Also known as the Königstein Fortress, this impressive structure is located near Dresden, in Saxon Switzerland, Germany, overlooking the Elbe River. One of the largest hilltop fortifications in Europe, it was used as a prison for centuries.

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Fort Douaumont, France -

Built close to Verdun in northern France in the 1890s, Fort Douaumont has a fascinating history, including being occupied by German troops during World War I.

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Chittorgarh Fort, India -

Also known as the Fort of Chittor, it was built near the Indian city of Udaipur in the 7th century. This imposing structure is the largest fort in India.

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Fort Knox, Kentucky, USA -

Built after World War I, the fort is home to the United States Bullion Depository. As you can imagine, security is second to none. It is said that the safe is capable of withstanding an atomic bomb attack.

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Crac des Chevaliers, Syria -

This Crusader castle was built by the Hospitaller Order of Saint John of Jerusalem between 1142 and 1271. This amazing fortress is a UNESCO World Heritage Site.

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Thule Air Base, Greenland -

Located just 800 mi (1,287 km) off the Arctic Circle, this air base is home to the US Air Force ground-based radar used for missile defense. It's also used for space surveillance.

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Maunsell Forts, England -

These towers, located in the Thames and Mersey estuaries, were built to protect England during World War II.

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Citadel of Aleppo, Syria - Situated at the crossroads of different trade routes of the 2nd millennium BCE, Aleppo has witnessed the rise and fall of a handful of empires and civilizations.

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Joint Defence Space Research Facility, Pine Gap, Australia -

Pine Gap is jointly operated by Australia and the United States. It's partly run by the US government agencies the CIA, NSA, and NRO. It's remote location makes it ideal to monitor satellites without spies intercepting the signals.

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Fortifications of Gibraltar, British Overseas Territory -

The fortifications on the Rock of Gibraltar were key for the British due to their strategic location in southern Europe, adjacent to the Strait of Gibraltar. It still is a British Overseas Territory to this day.

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The Alamo, Texas, USA -

The old missionary expedition turned fort in San Antonio is considered a shrine to the liberty of Texas.

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Malbork Castle, Poland -

A Catholic group called the Teutonic Order built the world's largest brick castle in 1406 on the bank of the Nogat River in Poland.

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Fort Jefferson, Florida Keys, USA -

Though it was never finished, this is still the largest brick masonry structure in the Americas. Featuring over 16 million bricks, the fort was built in the 1800s to protect the Gulf of Mexico from pirates.

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Prague Castle, Czech Republic -

Build sometime around 880, this is, in fact, a complex of buildings rather than just a castle. Prague Castle is the biggest coherent castle complex in the world, according to the Guinness Book of World Records.

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Mukachevo/Palanok Castle, Ukraine -

Built in the 14th century, this fortress was at one point used as a prison. It features over 130 rooms, where over 150 cannons were once placed. The castle also has a number of underground passages.

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Fort Murud-Janjira, India -

This amazing fort was built in the 15th century on an island in the Arabian Sea near Murud, in the Raigad district of Maharashtra, India.

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Navy Support Facility Diego Garcia, Chagos Archipelago -

The atol is used for logistical support for UK and US troops in the Middle East. It's located 1,000 mi (about 1,600 km) from India.

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Cheyenne Mountain Complex, Colorado, USA -

This complex, inside the mountains around Colorado Springs, is run by the North American Aerospace Defense Command.

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See also - The most incredible walled towns around the world

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