The Smithsonian Institution Building, often called the "Castle," is located on the National Mall in Washington, D.C. Designed by architect James Renwick Jr., it was completed in 1855 and serves as the main headquarters for the Smithsonian Institution. The building is known for its distinctive red sandstone and Romanesque architecture. Inside, the Castle houses information about the various museums and research facilities that are part of the Smithsonian. Visitors can find maps, guides, and exhibits that highlight the institution's extensive collections, which include art, history, and science from around the world.