Immortalized by artists, this church incorporates a medieval lighthouse into its shoreside structure.

This castle-fortress is a living example of the transition from medieval defenses to modern fortifications.

The former 1920s mansion of a prominent Catalan developer was used as the Soviet Embassy during the Spanish Civil War, and there is still a bunker below the garden.

A vista of the city and sea from atop the little-known viewing deck on an unassuming Columbus statue.

A once-secret library of Freemason history in Catalonia.

A former steam-powered mill from the 1800s is now an industrial park with a giant steel dragon slide.

One of Europe's largest supercomputers is housed inside of a disused chapel.

The 2,000-year-old Roman columns are hidden within Barcelona's Gothic Quarter.

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

A beautiful but toxic fountain of mercury.