The Polish capital has had a deep connection to its guardian mermaid for almost a millennium now.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.

The former Nazi Germany concentration camp stands as a museum to remember this dark chapter in European history.

An underground city of salt.

The oldest cemetery in New Orleans, resting peacefully for over 200 years now.

A castle from the Holy Roman Empire that hosts traveling youth... in its stables.

Monasteries on rock pillars, once accessible only by frayed ropes.

Bronzes and busts that once stood in the city's main squares are now relegated to a small hidden courtyard.

According to a local legend, this old castle could only be built after a woman was immured in the walls.

This ominous Albanian fortress houses tanks and downed planes as a reminder of their victories over Western imperialism.

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

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

An homage to the medieval rulers of Bulgaria.

Outstanding, eerie frescoes and decorations fill this 19th-century monastery.

This historic black and white monastery is one of Bulgaria's architectural jewels.

An abandoned saucer-shaped monument in the Bulgarian mountains.

This ancient Irish tower could easily have stood in for Rapunzel's prison.

A nun spent 16 years painting the stunning Celtic mural that covers the interior of this WWI memorial.

An 80-year-old plane tree is devouring an iron bench at Ireland's oldest law school.

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

This crumbling medieval basilica once attracted pilgrims by the thousands to collect a miraculous dust that formed above the saint's tomb.

This former Imperial summer home was the venue for a royal slap that once echoed 'round the world.

The ancient latrines of the Greeks, in all their public glory.

Ancient serpentine sacrifice holder that has stood the test of time.