This was the ORIGINAL Smithsonian, a donation by a European in the 1800's and by providence made it to the US to create what became the foundation of an American institution. (For those of you who don't know, there is no single Smithsonian Museum, but many buildings scattered across DC.) The castle is a beautiful piece of architecture which, alone, would be worth walking through. Its exhibits focus on the Smithsonian formation and impact since its inception. Surrounded by gardens and sitting more on the National Mall than any other building besides the Washington Monument, itself, you should start any museum journey here.