Spanish architects used seized Muslim gravestones to rebuild parts of the fortress.

The atmospheric ruins of a historic church can be found on a hill above the city of Oviedo.

This chest hospital with a dark, haunted past serves as a fitting set for horror films and a paradise for ghost hunters.

A contemporary art installation housed within a Gothic cathedral.

This abandoned, turn-of-the-century astronomy hub is now the subject of more photographs than it ever took.

This imposing modern citadel started out as a home renovation project.

12th-century holy complex shares land with a natural park full of heavenly waterfalls and raptors.

An abandoned Indian-inspired mansion.

The Gaudí-inspired building that was the start of Terrassenc modernism.

A Spanish artist builds his own castle.

This religious complex carved into a cliff is steeped in legends of miraculous stags and the Holy Grail.

Abandoned waterpark turned counter-culture mecca.

A curious carving hides on a marble column in the Mosque-Cathedral of Córdoba.

An exact replica of a colossal Olmec head sits atop a pyramid within a Madrid traffic circle.

The oldest Roman lighthouse still in use.

The little church's beautifully frescoed ceiling has earned it the nickname “the Sistine Chapel of Madrid.”

A site where “the ravages of the war are more striking…than any other place in contemporary Spain.”

This former charity hospital for elderly clergy houses some of Seville’s most treasured paintings.

Cursed swimming pool left abandoned is now a favorite of filmmakers and ghost hunters.

A once grand hotel now sits eerily abandoned in the Catalan countryside outside Barcelona.

The studio of Catalonia's most famous theatrical designer remains exactly as he left it.

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.

This mosque was based on Sub-Saharan Islamic architecture and created for the Senegalese Tirailleurs.