Living trees form the structure of an open-air palace.

A long spiraling walkway winds through the woods, and around a copper beech tree at the center.

This "floating railway" is one of the world's oldest monorail systems.

Ruins of a German Castle on top of a towering stone pillar.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

The house that took a plunge into the facade of a museum.

Inside the corridors of this flea market, visitors are sure to find some obscure and odd treasures.

This series of Baroque passageways in Vienna's cultural center mix old architecture with modern art.

Museum devoted to the artificial language of Esperanto.

Vienna's underworld of tunnels and subterranean rivers made an iconic appearance in a 1949 Orson Welles film.

The townhouse where the "Blood Countess" Elizabeth Báthory started her murderous career.

Apartment complexes built into huge repurposed natural gas tanks.

This museum promotes informed contraception by documenting its often secret past.

This cubist Vienna church building looks like some sort of alien stonehenge.

The resting place for anonymous victims of the Danube's dark waters.

This Austrian museum celebrates the work of famous forgers and their inimitable imitations.

Located right outside of Vienna, visitors can take in expansive views of an ethereal forest.

Dine amidst exotic plants at this former royal greenhouse.

Op-art architecture designed for happiness.

A spherical "micro-nation" in the heart of Vienna.

The topiary maze at the Habsburg hunting lodge.

Built to give thanks for the passing of a plague epidemic, this cathedral is a one-of-a-kind architectural hybrid.

The beautiful baroque library of the Hapsburg empire.

A memorial in the Tiergarten is a reminder of the suffering inflicted on homosexuals by the Nazi regime.