It's forbidden to dig within this ancient Roman fort hidden beneath a playground.

A striking memorial to the Jews of Leiden who were murdered during World War II is scattered throughout the city.

An unassuming rod in the Netherlands was once the old standard unit of length for the entire Dutch empire.

A trio of red, white, and blue stones embedded in the streets of Leiden are markers of medieval territory and law.

This 11th-century castle sits so low you can't see it behind the surrounding buildings.

This historic Dutch newspaper building is covered with media-themed details—and one unrelated oddity.

This Dutch art piece became a Russian icon.

Science meets wall art to highlight the beauty of famous mathematical equations.

An ancient Egyptian temple sits within a modern European museum.

This public library and its impressive collection of academic books have not changed since the 17th century.

Devious political maneuvering and four ancient, working telescopes are central to the world's second-oldest observatory.

A 'living museum' in Leiden, the oldest of its kind in the Netherlands.

The works of poetry's biggest names grace the walls of the city of Leiden, all carefully painted by hand.

A former plague hospital turned medical museum.

Thousands of well-dressed mummies below a monastery.

This triumphal 16th-century arched gateway leads to the oldest street in Palermo.

A study of Fascist era Art Deco design in Italy.

Why a column on the entrance is inscribed with a verse from the Quran is one of Sicily's many mysteries.

A plaque honoring a Hungarian hero of Italy is hidden in plain sight in Palermo.

This Arabesque Italian castle was built with a primitive, if ingenious, air-conditioning system.

A prime example of socialist modernist architecture, wrapped in a enormous fresco about the media.

In the summer of 1957, a massive architectural exhibition showed off the future of urban living in this German neighborhood.

Dedicated to the European Sinti and Roma peoples murdered during the Holocaust.

For patriotic Russians, this memorial honoring the Soviet soldiers who died liberating Berlin in 1945 is the most sacred location outside of Russia.