An underground city of ancient quarry tunnels hid thousands of Allied soldiers in WWI.

Natural mummies hidden in the crypt of a medieval French church.

This French "sea of rocks" is packed with stunning rock formations and its own medieval castle.

Legend has it this 11th-century castle was paid a visit from the devil.

These medieval ruins have survived since the 12th century, even providing shelter from the Nazis.

Resting on a hill overlooking the coast, these medieval ruins look completely deserted save for their restored chapel.

This 1,400-year-old abbey was once a powerhouse for music and intellectualism known across Normandy.

Despite centuries of architectural innovation, this French castle is being built like it's the 13th century.

This triumphal arch celebrating the Sun King's conquest of Lille is a masterpiece of military architecture.

Series of underground houses inhabited continuously for centuries.

A daring D-Day assault by U.S. Army Rangers is commemorated atop the high cliffs of this strategic point.

The opulent home that inspired Tintin's Marlinspike Hall and the adventures within.

An ancient ossuary in Rouen.

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

The castle was reconstructed to fit Kaiser Wilhelm II's romanticized vision of the Middle Ages.

Perhaps the most beautiful of preserved saints, with a little help from Paris.

An Orthodox church built by the Royal Family for the Russian community exiled to the French Riviera.

The birthplace of the ferocious William the Conqueror, this castle is awash in medieval history.

The Church of the Annunciation was carved into the face of a cliff, just like the rest of the town of Haute-Isle.

A 2017 crowdfunding campaign saved this historic castle from being destroyed.

A centuries-old complex of hand-dug stone chambers has sheltered locals in times of trouble.

The remains of 40,000 humans, meticulously stacked in a tiny churchyard shed.

A swath of France so devastated by war it is still forbidden to go there.

Trek into the depths of the cave where 17,000-year-old paintings exist alongside 400-year-old graffiti.