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

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

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

The bridge is a secret gallery of discarded gum that has been transformed into miniature masterpieces.

These wire animal sculptures commemorate the exotic inhabitants that once called the Tower of London home.

The commemorative stone column conceals a secret laboratory.

An architectural oddity hides in plain sight on this iconic London bridge.

The "lost friend” is the most famous of the six moai statues that were removed from Easter Island.

London's last remaining public soapbox site has seen speeches from Karl Marx, Vladmir Lenin, and George Orwell.

Scattered throughout London are ancient remnants of the city's former bounding wall.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

A mysterious shape catches the eye in this 16th-century painting.

Elaborately carved to showcase the glorious wealth that the age of exploration brought to the Portuguese empire.

A king's pet, a pope's gift, an artist's inspiration, and a tragic shipwreck.

An ancient and majestic fortress that played an integral role in Portugal's road to independence.

This unreal Portuguese palace looks as though it's made from a pile of different castles.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

The world's oldest bookshop still in operation.

Hidden beneath a beautiful overlook you’ll find the history of the city depicted in comic strip format next to a pay toilet.

In the Monastery of São Vicente resides a gallery devoted to a breathtaking collection of seashells.

When this former hospital pharmacy closed in 2013 it already looked like a museum, so they turned it into one.

Building owners can take out tiles for free at this repository for historic building materials.

Ancient catacombs form the foundation for this Rococo orgy of a church.