Climb a steep hill in 90 seconds on the second-oldest urban subway in the world.

This historic hotel preserves many treasures from its heyday hosting passengers on the Orient Express.

Nine centuries-old fortresses guard the walls of this medieval Romanian city.

Something's watching you.

This communist behemoth is the heaviest building in the world and a legacy of a brutal regime.

An homage to the medieval rulers of Bulgaria.

Rebuilt several decades ago, its striking murals prevent it from being reconsecrated as a church.

This ancient theater, restored in the 20th century, lives on as a venue for dramatic and musical performances.

A gigantic statue of a Russian soldier watches over the ancient city of Plovdiv, a prominent reminder of the Soviet period in Bulgaria.

An abandoned Byzantine fortress and notorious prison.

Stone age urban planning.

This 14th century castle withstood years of battle but is now sliding off the hills into the Danube.

Nikola Tesla's ashes are held in a golden sphere, surrounded by his works.

A misnamed well, with a deep dark past.

A towering spire in the fortress of Kalemegdan honors the victorious armies of Serbia during their struggles against the Ottoman Empire.

Religious perseverance and cultural coexistence manifest in an architectural gem that multiple wars couldn't topple.

This dazzling Art Nouveau mansion is now home to the Modern Art Gallery Subotica.

This region of Slovakia boasts some of the most plentiful opal mines in the world.

Hamburgers are intensely proud of their tunnel beneath the river.

Hamburg's first legal sex work is relegated to a gated street that still does not allow pedestrian women.

A museum that chronicles the history of the Nuremberg Trials for Nazi war criminals in the building where the trials were once held.

From Ancient Rome to WWII, Neupfarrplatz has over 2,000 years of history underneath its main square.

This riverside restaurant has been serving customers since 1146.

The prototype for all Nazi concentration camps.