These historic defenses ensured the city within would survive hundreds of years of occupations, sieges, and invasions.

This fairytale inspired, warped building houses restaurants, shops, and a radio station.

Beijing's version of Disneyland, completely abandoned, mostly destroyed.

Built to transport German U-boats through the never-completed Masurian Canal, the locks near this Polish village are now home to a rope course.

The concrete shell of an industry giant remains as a playground for graffiti artists, auto scrappers, and paintball warriors.

The ornate structure is part of a building boasting more marble than the Vatican.

This Nazi ruin was the site where Hitler was almost blown up by a suitcase bomb.

The New York hotel where tragic visionary Nikola Tesla spent his final hours, destitute and alone but for the pigeons.

Elevated freight railway turned wildly successful urban park.

Try not to get slimed in Tribeca.

This seemingly out-of-place gas station by the Watergate hotel was once described as the most expensive gas station in the world.

An engineering marvel at the time of its construction, this cross-border waterway sought to connect the Black Sea to the Baltic.

An old hill fort covered in over 100,000 crosses is a symbol of Lithuanian nationalism and beliefs.

Over time, this castle grew to encompass the entire island.

The first museum dedicated to exploring humanity's cultural relationship with the celestial world.

An old tram depot transformed into a vintage car paradise.

Started as a kitchen garden, now the second largest botanical garden in the world.

A whimsical bridge adorned with decorative dragons.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

A miniature train-ride through Slovenia's largest cave system.

A World War II evacuation tunnel turned into a history display of Croatian state and people.

This communist behemoth is the heaviest building in the world and a legacy of a brutal regime.

The railway carts travelers to central Athens's highest peak, which offers stunning vistas of the city.

Berlin's old airport.