This Hungarian castle was built out of cardboard and wood until it proved so popular that it got upgraded to stone.

A trail of iron footwear stands as a monument to the thousands executed along this riverbank during WWII.

This Budapest bar allows drinkers to leave their mark on the walls, floor, and even ceiling.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

A museum of carnival attractions where you are invited to ride the rides.

This massive library holds what was once the largest book collection in the world.

"In the Dark" Restaurant in Paris offers dinner in complete darkness.

Artists and musicians now rule over this former rail station.

This wildly-tiled artist's palace is a living and growing account of the creator's life, loves, and labors.

A 2000-year-old Roman amphitheater lies just below Guildhall Yard.

The popularity of Sherlock Holmes led to the creation of his fictional address, turning the orderly London street numbers askew.

A kaleidoscopic warehouse-maze of handmade neon signs.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

Art and a pun, all in one.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

A whimsical and twisted animatronic monster tour beneath the streets of Berlin.

An eerie, derelict amusement park in Berlin.

The world's only museum devoted to the wonders of fluorescence.