Remnants of the 16th-century defensive wall still stand unassumingly within Edinburgh.

Overlooked by many, the two famous Scottish warriors guard the main gate of Edinburgh Castle.

The 16th-century church has the only stained glass windows that survived the Scottish Reformation intact.

A book bound in the skin of an infamous criminal, along with countless other specimens.

There are plenty of wonderful panoramic vistas in Edinburgh, but not many are hidden smack in the middle of the Old Town.

The final resting place of soldiers' loyal canine companions.

A sweet symbol marks the place where torturous executions were once carried out.

Accessible only at low tide, the deserted island is dotted with World War II ruins.

17th century streets hidden under Edinburgh were once a breeding ground for the black death.

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

Graffiti from the 16th and 17th centuries cover the walls of this part of the Tower of London.

One of the figures in this Celtic sculpture continues to puzzle archaeologists.

A labyrinth hides in plain sight at each of the city's 270 Tube stations.

This innovative steam machine was no doubt the most famous of the pioneer locomotives.

The library houses an incredible collection of some of the world's most impressive books and manuscripts.

Tucked away in the Tower of London is the weapon once used to execute high-profile prisoners.

These ornate arches show how the course of the River Thames has changed.

The sumptuous ceiling was one of the last things Charles I saw before his execution.

Never actually a tree at all, this spot was the site of London's public hangings for nearly 600 years.

Tombstones and enigmatic inscriptions hide within a vault beneath a popular London arts venue.

These official units of measurement—including chains and perches—lie hidden beneath tourists' feet in a popular London hub.

The handiwork of a underdog clockmaker who solved an impossible problem for an Empire, and forever revolutionized seafaring.

Originally built for London’s cab drivers, this bright green hut now serves anyone a hot drink or meal.

The 2,000-year-old beam is tucked within the pedestrian entrance to the old London Bridge.