Frontier Fortress of Rammed Earth
Yumen Pass, or Yumenguan, is not a mountain pass but a square, mud-brick fortress in the Gobi Desert. Established during the reign of Emperor Wu of the Han Dynasty around 121-107 BC, it marked the western frontier of China proper. The primary remaining structure, known as the Small Fangpan Castle, measures 24.4 meters by 26.4 meters and covers an area of over 630 square meters. Its walls, constructed of rammed loess soil layered with reeds and tamarisk branches, originally stood about 10 meters high. This construction technique provided durability against the harsh desert climate, which sees temperature swings from -20°C in winter to over 40°C in summer.
The fortress was part of a larger defensive system. This system extended for about 45 kilometers and included 18 sections of ramparts and around 20 beacon towers. These towers, spaced every few kilometers, formed a communication network that used smoke and fire signals to transmit military alerts across the desert. Archaeological work at the site has uncovered thousands of bamboo slips, which were the primary medium for record-keeping. These documents provide detailed accounts of garrison life, administrative duties, and the movement of personnel and supplies.