This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

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.

Fortified homes of highland warriors that are still in use today.

An abandoned Byzantine fortress and notorious prison.

17th century palace once a meeting place for politicians.

This Transylvanian church has changed its denomination four times.

Religious structure known for its sculpted details.

This hauntingly beautiful cemetery outside a medieval citadel is full of winding paths and ivy-covered graves.

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

A fire once nearly devoured this gorgeous 14th-century Gothic cathedral.

The European towns of Brașov and Râșnov each have proud signs reminiscent of the Hollywood sign.

This snug medieval lane is one of the narrowest streets in the world.

Something's watching you.

One of the oldest churches in Romania is believed to have been built on the site of a pagan temple.

Once an opulent Art Nouveau hotspot, the Constanța Casino is one of Romania's most beautiful abandoned buildings.

The possible imprisonment of Vlad the Impaler here earned the medieval castle its nickname, "Dracula's Castle."

This elaborately decorated, fairytale-like Romanian royal palace is one of the most striking castles in Europe.

This massive underground salt mine now holds a sci-fi theme park in its depths.

Moss and stone turn this waterfall into an otherworldly liquid veil.

This fairy-tale castle would make Dracula (who is said to have been imprisoned there) jealous.

Near Belgrade, an ancient-looking sarcophagus serves as a World War I monument.

Imperial palace of Emperor Galerius, the Emperor behind the worst Christian persecutions during the Roman Empire.

A maze of underground tunnels beneath one of the most beautiful forts in Serbia.