Old batteries and frog skeletons reside in this lakeside monument to Alessandro Volta.

From Austrians to Nazis to Soviets, this historic Budapest fortress has a history as complex as the city itself.

This Hungarian castle was built out of cardboard and wood until it proved so popular that it got upgraded to stone.

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

This memorial to a murdered empress took 20 years to design, then spent 30 years hidden away in storage.

France's high-tech, future-themed amusement park.

The opulent home that inspired Tintin's Marlinspike Hall and the adventures within.

Despite centuries of architectural innovation, this French castle is being built like it's the 13th century.

A fortified palace in the shape of a ship on the Thiou river.

This castle-fortress is a living example of the transition from medieval defenses to modern fortifications.

The site of the first medieval crusade that pitted Christians against Christians.

Historic medieval fortified city in southwest France.

This French castle is said to have once held the Holy Grail but is now a gorgeous mountaintop ruin.

There is a strange little carving of a person seemingly crushed by a pillar behind the remains of St. Thomas Aquinas.

A medieval church with largest and oldest collection of Renaissance frescoes in France.

Ancient paintings known as the "Sistine Chapel of Cave Art."

Explore an eerie, abandoned medieval citadel.

This impressive chasm is one of the longest natural underground cavities in the world.

A 14th century bridge built as a fortress, where the devil lurks on one of its towers.

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.