A lone cannonball remains lodged in the walls of the Hotel de Sens as a sober reminder of a revolution.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

The first green space constructed on an elevated viaduct.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

The ruins of a Duke's bucolic dream, where camels once roamed alongside "Dutch" windmills and faux-Italian vineyards.

This Parisian fountain is a little bit fantasy and a little bit nightclub.

This Parisian café sticks almost exclusively to an ethos of reuse.

Paris' fabled purveyor of exotic taxidermy and natural history curios.

Known as the "Dodo Manège," this carousel features a herd of animals that have disappeared—or may be gone soon.

Indents in the pavement are all that remain from a grisly death machine in Paris.

Notorious artist squat renovated into legal studios.

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

This cafe offers a curated collection of treats and tipples made by monks and nuns.

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.

Stare out over the lagoon and beyond from this 10th-century campanile on the island where the city of Venice began.

This Roman statue is missing a limb from when the Venetians stole it from Constantinople during the Crusades.

Originally a leper colony, the island is now an Armenian monastery that's home to an incredible manuscript library and eclectic museum.

Hanging within this medieval church are the bones of a beast allegedly slain by a fourth-century saint.

Thousand-year-old Viking runes adorn an ancient Greek lion at the entrance to the Venetian Arsenal.

Where Venice's gondolas go for a tune-up.

Those who own or stay at this 500-year-old, allegedly cursed house often meet terrible fates.

A sinister pyramid in the Frari contains the heart of the famous neoclassical sculptor.

The small peaceful island off the Venice tourist circuit is where the city began more than 1,500 years ago.