In the days before house numbers, these colorful signs held clues about various buildings' purposes.

Europe's official "zero-level," the equator of elevation.

The former home of a mad diplomat is said to still be marked in bloody, arcane scrawls.

Amsterdam's former hotspot for public executions and dissections.

One of Amsterdam's narrowest houses is located right across the canal from its widest.

Three connected pink triangles memorialize persecuted gays and lesbians throughout history.

A Dutch passageway's artistic, aquatic-themed makeover pays tribute to the capital city's famed waterways.

The attic of a four-century-old canal house hides a clandestine church.

This 22-floor tower's Skydeck offers panoramic views of Amsterdam.

This historic Amsterdam bar used to accept sailor's monkeys as payment.

An ancient passage filled with stalls selling used books, once frequented by Vincent Van Gogh.

A curious collection dedicated to human and animal anatomy and pathology.

The world's first zoo for microbes collecting the horrible and wonderful things living on and around us.

This gorgeous reading room houses the oldest and largest art history library in the Netherlands.

Cat art as far as the eye can see.

Established in 1638 to battle the Black Death, Hortus Botanicus Amsterdam is a treasure trove of rare flora.

Museum preserving torture methods from the past.

A floating cat sanctuary on an Amsterdam canal.

An Amsterdam street lined with antique shops each one dedicated to a different obscure collection.

Six ravens are kept captive (but well-fed) at the Tower of London to prevent the fall of the Crown.

Abstract dinosaur models in London.

An alleyway plaque is all that remains of the independent Texas' envoy that forgot to pay their rent for over a century.

A hands-off space for graffiti artists to hone and display their skills.

An unusual round church in London with a Templar past.