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

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

A mud statue stands in the dark tunnels, seeming to hold up the weight of the city above.

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

The tradition of open-air secondhand and antiquarian bookselling in Paris dates back to the Renaissance.

The narrowest street in Paris.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

This demon atop the Notre-Dame de Paris looks utterly bored.

The celebrated eyesore created by starry-eyed sweethearts.

A simple plaque on the facade is the only reminder that the treaty ending the American Revolution was signed inside.

This bridge was constructed from the rubble of the infamous Bastille.

Twelve grave-faced marble angels guard the emperor's tomb at Les Invalides.

An unusual number plate signifies the building's saucy past.

A surrealist timepiece by the artist of melting clocks.

The Swiss artist's bulbous and overgrown shape-shifting sculpture controversially graces the center of Bern.

A nearly 500-year-old sculpture depicts a man eating a sack of babies, and no one is sure why.

This simple apartment may have inspired some of the most important theories in the history of science.

This memorial commemorating fallen Swiss Guards of the French Revolution has been called the saddest stone in the world.

Lucerne's central prison was converted into a tourist hotel with a bare minimum of changes.

This historic covered bridge contains a series of macabre paintings that communicate one thing: everyone dies.

This artistic fountain display pays homage to this area's theatrical history.

The second-highest train station in Europe.

Just across from the "Last Supper" painting is the famous painter's vineyard, a gift received for creating the masterpiece.

The mark of the Devil's horns on a marble column in Milan.