This small, ancient Roman temple was once a church dedicated to the patron saint of penitents.

The final resting place of the great navigator, or maybe his brother.

The world's largest wooden structure.

This medieval synagogue survived the Inquisition as a hospital and church.

Spain's unique Mosque-Cathedral has been a place of worship for both Muslims and Christians throughout its history.

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

This magnificent castle grew from a small Moorish fortress, and reportedly inspired two iconic Disney castles.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

Housed in a historic 1840 home, this museum provides a window into the lives of everyday, 18th- and 19th-century Bulgarians.

This fifth-century church is one of the oldest Christian sites in the Balkans.

An homage to the medieval rulers of Bulgaria.

A massive cave once a secret military base is now home to thousands of bats.

Sofia's legislative center is continuously reinventing itself.

Rebuilt several decades ago, its striking murals prevent it from being reconsecrated as a church.

"Holy Wisdom" overlooks the city that became its namesake.

Underneath this Byzantinian church lies a necropolis filled with ancient tombs and the ruins of previous churches.

This ancient theater, restored in the 20th century, lives on as a venue for dramatic and musical performances.

A striking symbol of the Bulgarian capital.

This massive concrete sculpture is said to be the heaviest communist monument on Earth.

Bizarrely shaped rock formations in northwest Bulgaria come with equally bizarre legends.

An abandoned saucer-shaped monument in the Bulgarian mountains.

A castle from the Holy Roman Empire that hosts traveling youth... in its stables.

Vienna's diversity-themed traffic lights were inspired by the Eurovision Song Contest.

The beautiful baroque library of the Hapsburg empire.