The claim that the Wall is 300 miles long is further supported in Game of Thrones: A Telltale Games Series, where Britt Warrick says to Gared Tuttle a 300-mile wall, and you're still in my way. The game is canon to the show so it is possible that Sam was wrong.
One of the most respected collections of museums in the world, the Smithsonian Institute is an inspiring attraction for kids, teens and adults. Comprised of 19 different museums, art galleries and research centers, the Smithsonian collection is centered around the National Mall - and admission is always free. The most popular Smithsonian museums include the National Archives, National Zoo, National Museum of Natural History and the National Air and Space Museum.
This Victorian style, red sandstone building was built in 1855 and designed by architect James Renwick, Jr. It was originally the home of the first Secretary of the Smithsonian, Joseph Henry, and his family and is the oldest building on the National Mall. The Smithsonian Castle is centrally located on the National Mall and serves as good place to start a tour of the Smithsonian museums. You can view a 24-minute video on the Smithsonian and learn about other Washington, DC attractions as well.