This Industrial Age architectural and engineering gem helps visitors and residents alike traverse Lisbon, “The City of Seven Hills.”

An outsider folly near Madrid.

A huge cathedral single-handedly constructed by one eccentric monk.

Madrid's romantically ornate crystal palace has been offering shelter to plants and art for over a century.

An ancient Egyptian temple in the middle of Madrid, Spain.

Commonly acknowledged as the only public monument to the Devil himself.

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

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

The internal railway of one of the biggest steel plants in America has been transformed into a linear park with unique industrial views.

This quaint Hungarian railway system is operated exclusively by children between the ages of 10 and 14.

One man's personal collection of KGB memorabilia is presented along with his strange viewpoint.

Dozens of musicians have planted trees in this Danish park, and donated songs to go with them.

The remains of a grand castle that may have inspired Bram Stoker's Dracula.

This bronze statue was erected to honor a small Scottish town's beloved stray cat.

The ruins of this gorgeous clifftop castle look like something from a fantasy novel.

Scotland's oldest medieval bridge is also among the last reminders of early medieval Aberdeen.

An intriguing three-sided folly is a testament to one man's devotion to Catholicism and the number three.

A Hungarian pharmacy museum with a box full of mummy powder.

A random tree outside a Budapest hotel has become a DIY shrine to the late singer.

A trail of iron footwear stands as a monument to the thousands executed along this riverbank during WWII.

Museum dedicated to the terror regimes of Hungary.

Sitting beneath an unremarkable residential building is a museum devoted to classic Pinball.

This Hungarian castle was built out of cardboard and wood until it proved so popular that it got upgraded to stone.

Below a historic English market town lies a secret underground city complete with kitchens, laundries, storerooms, and an underground lake.