100-windowed castle turned art gallery, built by one dedicated mason.

Hundreds of stone stairs and a winding medieval bridge connect this haunting Spanish island to the mainland.

Giant topiary dog once played a role in a terrorist bomb plot.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

The largest Art Nouveau complex in the world is a Barcelona hospital with a 600-year-long history.

Step inside the first house designed by Catalan architect Antoni Gaudí, considered to be one of the first Art Nouveau buildings.

Monsters and miniatures are held in equal esteem at this wondrous museum.

Lyon, France is home to two remarkably preserved Roman theaters, one dating back to 15 BCE.

For hundreds of years, automatons moving to metallic music have been marking the passage of time.

Lyon's miniature version of the Eiffel Tower was built as a statement in support of French secularism.

Lyon's "Wall of the Silk Weavers" is one of the largest murals in Europe.

Hundreds of hidden passageways weave through the old quarters of Lyon.

You'd never know the houses at 23-24 Leinster Gardens were fakes—until you see the train tracks on the other side.

This luxury shopping lane has been protected by its own tiny police force for over a century.

The largest of London's subterranean rivers and once a mad, bad center of London life.

After nearly 60 years, there's still a bear at Paddington Station looking for help.

An unusual round church in London with a Templar past.

This 1920s Dutch barge is now a floating bookstore.

Banksy gives themself artistic license with this piece of London street art.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

A 300-year-old tea shop that brought tea to the English people, not to mention the Queen herself.

A kaleidoscopic warehouse-maze of handmade neon signs.

The celebrated eyesore created by starry-eyed sweethearts.

During the summer months, giant teddy bears take over Paris near Les Gobelins metro station.