The Smithsonian Institution"s signature Building, popularly known as the "Castle," was designed by James Renwick, Jr.  It was the first Smithsonian building.  Renwick's, other works include St. Patrick's Cathedral in New York City and the Smithsonian's Renwick Gallery.  The "Castle" building is constructed of red sandstone in the Norman style.  When it was completed in 1855, it sat on isolated land cut off from downtown Washington, DC, by a canal. In the ensuing decades, the Castle became the anchor for the National Mall, as additional museums and government buildings were constructed around it.

Over the years several reconstructions have taken place. The first followed a disastrous fire in1865. In 1883, the east wing was fireproofed and enlarged to accommodate more offices. Remodeling from 1968 to 1969 restored the building to the Victorian atmosphere reminiscent of the original .

The Castle served as home and office for the first Secretary of the Smithsonian Joseph Henry. Until 1881, it also housed all aspects of Smithsonian operations, including research and administrative offices; lecture halls; exhibit halls; a library and reading room; chemical laboratories; storage areas for specimens; and living quarters for the Secretary, his family, and visiting scientists. In 1881, the US National Museum, now known as the Arts and Industries Building, opened adjacent to the Castle to house most of the museum collections.

The Smithsonian benefactor , French-born, British-raised James Smithson,  never set foot in Washington, DC, but he has had a huge impact on the city.  This illegitimate son of a Duke, left his considerable fortune to a nephew with the stipulation that were the nephew to die without heirs, the money would go “to the United States of America, to found at Washington, under the name of the Smithsonian Institution, an establishment for the increase and diffusion of knowledge.” The nephew died, and a museum complex was born.  It remains a mystery why Smithson left his money to the U.S, but theories abound. The items to be exhibited were specifically confined to the fields of natural history, art and objects of "foreign and curious research."

In 1977, the Castle was awarded Historic Landmark status.  Today, the Castle houses the Institution's administrative offices and the Smithsonian Information Center. Located inside the north entrance is the crypt of James Smithson, benefactor of the Institution, while outside on the Mall, a bronze statue of Joseph Henry honors the eminent scientist who was the Institution's first Secretary.