The largest self-built castle in the U.S., six decades in the making.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

This extinct volcano rises above Edinburgh and is special for its panoramic views over the city.

A whimsical bridge uses saxophones to pay tribute to two great men and the European Union.

Rare wooden escalators lead deep down into an otherwise staggeringly monotonous tiled tunnel.

This narrow Ghent alleyway is an officially sanctioned street art tunnel.

This giant urban staircase was built to keep the city's brave fighting men from getting waylaid in the red light district.

The tallest mountain in mainland Netherlands isn't actually very tall.

This impressive collection of rare books on ancient mysticism inspired author Dan Brown.

You can see three Irish counties from this Neolithic hillside fort, once the seat of the Kingdom of Ailech.

This "zoo" is a cross between a junkyard and a sculpture garden, featuring animals you won't see anywhere else.

Legend says this sacred tree was a gift from goddess Athena.

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

Hideout of thieves, monks, and goat-footed gods.

The oldest skyscraper of Europe now stands dwarfed by the surrounding structures.

Rotterdammers' dirty minds gave this Santa a dirty nickname and cursed it to a life of wandering.

Birthplace of the "Cat in the Cradle" and a cluster of iconic, original windmills that let the Dutch tame the seas.

Underground attraction offering tours of the foundational ruins of a church nave that collapsed in 1674.

This large sand dune in the Netherlands is home to wind-whipped trees with "floating" roots.

In the Netherlands, one of the world's largest private car collections can be found.

An Amsterdam School landmark combining a museum and social housing.

Artwork tucked within the trees greets visitors with surreal sonic experiences.

The 20-mile causeway that separates the Netherlands from the sea.

Unsurprisingly, Holland is home to one of the most magnificent flower gardens in the world.