An expansive complex of subterranean tunnels is all that remains of one of Europe's most formidable medieval fortresses.

The last bastion of Roman presence north of the Alps looms over a German town.

A trail of iron footwear stands as a monument to the thousands executed along this riverbank during WWII.

The comedy and tragedy of Oscar Wilde is preserved in a Dublin park, captured in a sculpture of colorful stone.

The somber sculptures commemorating Irish Famine on the river Liffey in Dublin.

The modern birthplace of Irish freedom.

The Habsburgs' royal collections of art and wonders.

An extraordinary "City of Caves" carved from solid stone.

Mummified monks and the accidentally interred, in a 17th-century crypt.

A cheeky architectural easter egg.

Europe's second largest ossuary wasn't discovered until 2001.

A peculiar clock commemorates a historic victory won by ringing the noonday bell an hour early.

On the Spree bank, discover one of the last reminders of a symbolic wall that divided, and was torn down by the people.

Macabre astrological automaton clock dating to the late 1400s.

Berlin's old airport.

Op-art architecture designed for happiness.

Vienna's giant wonder wheel had a starring role in one of the greatest noir movies of all time.

The former private quarters and office of Dr. Sigmund Freud now hosts a museum housing the works of the founding father of psychoanalysis.

Macabre museum devoted to historical Viennese murders.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

The solemn exterior conceals an opulent interior of this 17th century church.

Baroque celebration of the end of the Great Plague of Vienna.

This Nazi-built amphitheater was constructed on a sacred mountain site used by various German cults.

Sitting in the basement of a castle, the largest wine barrel in the world is bigger than most apartments.