Known to the Chinese as "Changcheng" (long wall), we visited the Badaling section nearby to Beijing.  It is popular because the tourist can take a cable car up to this section which has been restored.  Sections of the wall are very steep as it traverses the mountains, but it's easier to start from the top and walk down.  We walked along about a mile and a half of the wall, and it's hard to imagine how it could have been built in such inaccessible terrain over 2,000 years ago.