VTcat, That was right next to the Vietnam memorial wall. Be sure to see all the monuments, especially WWI and WWII. Bring a lot of film if you have a film camera. WWI thing is pretty hidden, but cool. The castle building is the history of the Smithsonian, and when it all fit in there the original building. I had no time to go in it but we took a tour that explained stuff and showed us the things we didn't have time to fully see. I really recoment a tour if you only have a short time and/or want to come back because it gives you a good idea of what to see next time.