This reconstructed ancient fort was home to one of the few horse training rings in the Roman Empire.

The ruins of this medieval tower are destined to crumble into the sea.

A palace-fortress ruin in Indonesia complete with legends of two magical tigers, eight bathing angels, and a wish-granting cannon.

This medieval citadel once guarded the western entrance of the Pamir Highway.

An ancient iron pillar in Delhi that seems to be rustproof.

The former Willingdon Airport started operations in 1929, in India's capital, Delhi. Now, it's used for maintenance runs and helicopter rides.

This ruined relic of the medieval city of Jahanpanah is a quiet oasis in the middle of New Delhi.

The castle was reconstructed to fit Kaiser Wilhelm II's romanticized vision of the Middle Ages.

A maze of underground tunnels beneath one of the most beautiful forts in Serbia.

A well preserved medieval fortress in Vojvodina.

A WWII museum that tries to reenact history.

A line of disused military bunkers built to defend against Nazi invasion lies hidden on the outskirts of the city.

Remnants of the Nazis' occupation of Denmark linger along the sand.

A Victorian Fort built into the chalky cliffs that overlook the English Channel.

The first English fort built in India.

A 14th century castle built on the seat of a mighty giant.

This volcanic plug boasts the longest recorded history of any Scottish stronghold.

This Arab fortress was once the largest citadel in Western Europe.

The little-known second castle that William the Conqueror built in York.

This 11th-century castle sits so low you can't see it behind the surrounding buildings.

This castle has links to Portugal’s founding and struggles for independence.

Legend has it fairies gathered at the site of this medieval castle.

Gladiators battle and chariots race daily in the ruins of a once-great Roman city.

Mosaics, storks, and brothels—this ancient city has it all.