These ruins reveal the remarkably intact layout of a Roman marketplace.

The site of the first medieval crusade that pitted Christians against Christians.

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

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

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

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

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

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

An ancient monastery, a locked room, and a mysterious book heist.

Lyon, France is home to two remarkably preserved Roman theaters, one dating back to 15 BCE.

A mosaic-tiled house built by graveyard sweeper Raymond Isidore.

This stunning French estate was so lavish that it landed its original owner in jail for the rest of his life.

A public park off the beaten path with a beautiful waterfall and 13th century Cistercian abbey.

A well-preserved medieval village nested on a rocky outcrop and surrounded by a network of cliffs, natural bridges and canyons.

The famous winding walk at Chartres is surrounded in mystery and myth.

Historic medieval fortified city in southwest France.

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

This historic island prison holds one of the dungeons from the Count of Monte Cristo even though it is a work of fiction.

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

Hundreds of hidden passageways weave through the old quarters of Lyon.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

"In the Dark" Restaurant in Paris offers dinner in complete darkness.

This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.