Wedged between streets and buildings, this ancient Roman theatre could seat 7000 spectators.

This is one of the very few intact windmills built by the Knights of St. John in the island nation of Malta.

These clocks tell time in a theological sense.

This spectacular dome managed to survive after being bombed in World War II, in an event known as the "Miracle of Mosta."

In the past, Renaissance sex workers were encouraged to entice customers on this bridge.

This once-derelict building is now one of the most beautiful bookstores in the world.

The Habsburgs' royal collections of art and wonders.

The largest Art Nouveau complex in the world is a Barcelona hospital with a 600-year-long history.

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

Spanish architects used seized Muslim gravestones to rebuild parts of the fortress.

Before it was buried by volcanic ash, this fast-food restaurant fed the hungry hordes of ancient Rome.

The stunning interior of this Gothic church is covered in ornate gold details.

This $62 million dollar temple was erected to house one little dental fragment of religious history.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

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

These mysterious Viking chess pieces spent centuries hidden on a remote Scottish island.

A spectacular Petrine Baroque summer palace commissioned for Catherine I by Peter the Great.

A former imperial sea-fortress and Soviet prison.

The ruins of an Estonian prison are drowning in the quarry lake where the convicts were once forced to work.

A musical instrument played by the sea.

Khmer Rouge's high security prison.

One of Amsterdam's narrowest houses is located right across the canal from its widest.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.