Simatai

 monument, wall(s), invisible, UNESCO World Heritage Site

A section of the Great Wall of China located in the north of Miyun County, 120 km northeast of Beijing, a strategic pass in the eastern part of the Great Wall. It had been designated UNESCO as one of the World Cultural Heritage sites.
Simatai, (simplified Chinese: 司马台; traditional Chinese: 司馬臺; pinyin: Sīmǎtái) a section of the Great Wall of China located in the north of Miyun County, 120 km northeast of Beijing, holds the access to Gubeikou, a strategic pass in the eastern part of the Great Wall. Originally built during the Northern Qi dynasty (550-577) and rebuilt in the Hongwu years of the Ming dynasty by Qi Jiguang, this section of Great Wall is one of the few to retain the original features of the Ming dynasty Great Wall.
Simatai Great Wall is 5.4 km long with 35 beacon towers. This section of the Great Wall incorporated the different characteristics of each section of the Great Wall. A specialist on the Great Wall, Professor Luo Zhewen, has said "The Great Wall is the best of the Chinese buildings, and Simatai is the best of the Great Wall."