The buildings of the Smithsonian Institution not only contain impressive collections; they are themselves icons of great cultural significance, many of them part of the historic National Mall. The Smithsonian's unique buildings illustrate the changing styles and sensibilities of America as an evolving nation. Representing the work of major architects, each building evokes a specific time in history:  the mid-19th-century turreted Castle, the sky-reflecting mid-century modern Air and Space Museum, and the golden, undulating, 21st-century American Indian Museum.