This 1855 Medieval-style building, with its eight crenellated towers and rich red sandstone facade, lives up to its nickname, at least from the exterior. Its Great Hall interior is rather unattractive, but that doesn’t matter, because you’re just here for information, possibly restrooms, and perhaps a bite to eat. 
There’s not much else in this big building that’s open to the public. The remains of Smithsonian benefactor James Smithson are buried in that big crypt in the Mall-side entrance area, which includes a small exhibit about the man. The pretty south-side entrance has been repainted to appear as it did in the early 1900s, when children’s exhibits were displayed here. On the east side of the building is the Castle Café, which opens at 8:30am, earlier than any other building on the Mall. Coffee, pastries, sandwiches, and even beer and wine are sold. Situate yourself at a table inside, where there’s free Wi-Fi, or outdoors in the lovely Enid A. Haupt Garden, and plot your day.