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

One of the most well-preserved castles in Italy also contains a rare medieval example of a port fortification.

This huge metal monument is slowly being stripped to its concrete skeleton.

Croatia's towering concrete blossom grows from the site of a World War II concentration camp.

This 135-year-old weather station studied the effects of the climate on human health.

This unique abacus hanging in the Technical University of Munich actually tells the time.

A line of abandoned, alien-like fortresses dot the mountains of North Italy.

This medieval castle was home to a known tyrant and features several hidden passages.

Featuring the last fully walkable parapet walkway in Europe.

A star-shaped fortress town fort built in the geometric model of the Renaissance utopian city.

Double spiral staircases are not unheard of, but they are very rare.

Resting in the middle of an Austrian river, this sci-fi "island" lets people cross in neon style.

The shell of a medieval fortress offers commanding views of the area it once protected.

The world's largest squirrel statue clutches a pecan bigger than your head.

Four-story slides send people whizzing through the math department at this Munich university.

Walk inside a half mile long crevice right outside the town of Garmisch Partenkirchen.

Destroyed numerous times, this centuries-old bridge has always been rebuilt with its original design.

A unique two-level bridge designed to honor a king.

A crumbling World War I fort that is reached through a series of concrete tunnels.

The monument honors the mountaineers who were part of Europe’s best anti-Nazi forces.

"World machine" built by an untrained Austrian farmer.

Brass plaques embedded in pavement preserve the memories of those who lost their lives at the hands of Nazis during World War II.

A unique hotel for the artistic soul.

A line of disused military bunkers built to defend against Nazi invasion lies hidden on the outskirts of the city.