The abandoned ruins of a beautiful medieval castle just outside Barcelona.

A gravity-defying "devil's bridge" that has a surprising architectural feature for its stability.

The blowhole bursts spectacularly right next to the buildings in a medieval seaside town.

This memorial, built by fascist dictator General Franco in 1960, has been repurposed by the people of Catalonia.

Long tunnels run under the ancient chariot racing circuit to the tower.

This Roman arena once played host to bloody gladiatorial battles.

Stunning Roman aqueduct used for 1900 years and still standing.

An open-air "garden" of mural art that changes weekly.

Medical museum that houses most notably the taxidermied remains of Vladimir Demikhov's famous two-headed dog experiment.

The world's first invisible city square is an homage to a man who pushed the boundaries of art.

An abandoned masterpiece of Soviet Brutalist architecture.

A former imperial sea-fortress and Soviet prison.

Rusting ships sit in a desert where a sea used to be.

This "tower of death" has acted as an observatory, a religious hub, and an executioner's lair.

Touching displays of wishes for peace from the Cold War era.