A self-built castle made by a mysterious man.

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

One critic described it as "middle Mussolini, early Ramses, and late Neiman-Marcus." Another called it an architectural "natural disaster."

The grand marble structure next to the White House is Andrew Carnegie's temple to Pan-American diplomacy.

D.C. once had its own tiny rival to the New York Stock Exchange.

A derelict bit of infrastructure from the canal that once ran through D.C. is landlocked in the heart of the city.

This mysterious structure by the Washington Monument is a flood barrier designed to protect the White House against rising waters.

A portal into the bricked up canal that runs through the heart of Washington D.C.

A muscular Art Deco monument represents the struggle between regulators and unbridled markets.

One of the most influential presidents in U.S. history wanted only this plain, elegant monument as his lasting memorial.

A diminutive memorial marks the site of a successful assassination by a right-wing death squad in America's capital.

Look closely and you’ll notice that the color changes a third of the way up the tower.

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

While most wind tunnels test scale models, the "Cave of Winds" was large enough for actual airplanes.

The little-known, 300-year history of the area includes former lives as a bustling tobacco port, parking lot, and industrial dump.

A bas relief commemorates the "Nuns of the Battlefield" who cared for soldiers on both sides of the conflict.

A tiny monument to the unsuccessful attempt by Thomas Jefferson to place the prime meridian in Washington.

The fountains of the U.S. Navy Memorial are annually salted with water from the Seven Seas as part of a nautical ritual.

Before Nixon, "watergate" meant canals.

A much-maligned monument to teetotalism.

This robust fence in front of a historic Georgetown home is likely made from hundreds of recycled Revolutionary War firearms.

A small triangle of DC grass is the final resting place of one of Sonny and Cher's songs.

Legend has it that President Grant’s frequent drinking in the lobby gave rise to the term “lobbyist.”

Washington's iconic carousel has a nice piece of Civil Rights history.