When you see the impressive Smithsonian Castle, it's easy to imagine that it must be an amazing museum of some sort. If you had, infact, visited it back in the 19th century after it first opened to the public in 1855, you would not have been disappointed. Today, however, it's more of a central hub for administering and promoting the wide range of Smithsonian institutions and museums. It often surprises people to realize that the National Museums of the United States are collected under the umbrella of the Smithsonian name. While most are located in DC, some are not, like the Cooper Hewitt Museum of Design in New York City. While most are thematic and function as repositories of precious, unique and interesting artifacts and artwork, at least one isn't--The Smithsonian National Zoo.
What you will discover when you enter is that a lot of the space is not open to the public. The great hall is and basically hosts a small gift shop, a little self-serve cafe with ample seating and a few artifacts. An adjacent lesser hall contains displays illustrating the history, organization and function of the Smithsonian Institution itself.
The largest of the gardens with a beautiful formal lawn criss-crossed with little hedgerows and flowers. It frames the castle like an English country estate. The Moongate Garden occupies a large portion of the western side, and the eastern side is divided between formal and organic paths. A patio with tables and umbrellas defines the easternmost edge up against the Arts and Industry Building. The most famous of the focus points is the monumental Andrew Jackson urn; however, a little side water feature with a whimsical "coral reef" made of pumice rock, succulents and ceramic fish was my favorite. 
The Smithsonian Castle is an iconic structure roughly in the middle of the Southern side of the National Mall. It is surrounded in the larger sense by the greatest collection of museums and sculpture Gardens in the world, in my opinion. Up close, between the castle itself and the nearby Arts and Industries Building, it nestles among a collection of beautiful garden spaces.
