雁门关
The restorer Emperor Guangwu (r.25–57AD) initiated several projects to consolidate his control within the frontier regions. Defense works were established to the east of the Yanmen Pass, with a line of fortifications and beacon fires stretching from Pingcheng County (present-day Datong) through the valley of the Sanggan River to Dai County, Shanxi. By 38 AD, as a result of raids by the Xiongnu further to the west against the Wei River valley, orders were given for a series of walls to be constructed as defences for the Fen River, the southward course of the Yellow River, and the region of the former imperial capital, Chang'an. These constructions were defensive in nature, which marked a shift from the offensive walls of the preceding Emperor Wu and the rulers of the Warring States. By the early 40sAD the northern frontiers of China had undergone drastic change: the line of the imperial frontier followed not the advanced positions conquered by Emperor Wu but the rear defences indicated roughly by the modern (Ming dynasty) Great Wall. The Ordos region, northern Shanxi, and the upper Luan River basin around Chengde were abandoned and left to the control of the Xiongnu. The rest of the frontier remained somewhat intact until the end of the Han dynasty, with the Dunhuang manuscripts (discovered in 1900) indicating that the military establishment in the northwest was maintained for most of the Eastern Han period.