There are a few of the original exhibits that have been there a very long time. I found the architecture to be the most interesting thing about the castle.
The Castle has a cafe, nice bathrooms, maps, and the giant marble memorial urn of Mr. Smithson. There are small displays from each of the 19 museums that give an idea of each museum's specialty. The Castle wasn't a "must-see" destination for me- although the building's architecture is magnificent.
The small scale model of the city of DC
There's a cool map of the area with all the Smithsonians highlighted. There's also a cafe and store.
They have selected items from various Smithsonian Museums and the zoo, to showcase at the visitor center.
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