This gallery commissioned by Cardinal Spada is known for its magnificent optical illusion.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

The best preserved Paleochristian basilica in Rome hides a number of oddities and secrets.

This is the oldest Roman-made bridge in the city still existing in its original state.

In Rome’s Jewish ghetto, a 200-year-old bakery still makes the neighborhood’s historic sweets.

The only remaining arch of the oldest stone bridge in Rome stands in the middle of the Tiber.

This curious, round temple is the oldest still-standing marble building in Rome.

This ancient Roman prison is decorated with an upside-down cross not as blasphemy but as saintly tribute.

Charles Dickens was not a fan of the ghastly wall dressings in this 6th century church.

Its interior is adorned with some of the best examples of early Christian mosaics.

A fortified medieval basilica on the Caelian Hill preserves extraordinary frescoes and one of Rome's most beautiful hidden cloisters.

The spot where the fascist dictator made some of his most famous speeches including declaring war on Britain and France.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

A Nesting Doll of Churches.

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

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

This ancient market is often referred to as the oldest shopping mall in the world.

A forgotten "city of water" underneath the famous Trevi district.

Installed in an act of one-upsmanship, the annual sundial in this church is also a symbolic victory for the Gregorian calendar.

A labyrinth of ancient tunnels hidden below the city holds the ruins of 2,500 years of history.

This 17th-century passageway is home to numerous bookshops, a witch legend, and the oldest pizzeria in the world.

This shining multicolored metro station deep under Naples is said to be the most beautiful in Europe.

In a city that boasts four separate inclined cable railways, this is one of the busiest funicular lines in the world.

This magical-looking island is notorious for the misfortunes suffered by its owners.