<P> Following the success of the Ordos walls, Yu Zijun proposed construction of a further wall that would extend from the Yellow River bend in the Ordos to the Sihaiye Pass (四海 冶 口; in present - day Yanqing County) near the capital Beijing, running a distance of more than 1300 li (about 700 kilometres (430 mi)). The project received approval in 1485, but Yu's political enemies harped on the cost overruns and forced Yu to scrap the project and retire the same year . For more than 50 years after Yu's resignation, political struggle prevented major wall constructions on a scale comparable to Yu's Ordos project . </P> <P> However, wall construction continued regardless of court politics during this time . The Ordos walls underwent extension, elaboration, and repair well into the 16th century . </P> <P> With the Ordos now adequately fortified, the Mongols avoided its walls by riding east to invade Datong and Xuanfu (宣 府; present - day Xuanhua, Hebei Province), which were two major garrisons guarding the corridor to Beijing where no walls had been built . The two defence lines of Xuanfu and Datong left by the Northern Qi and the early Ming had deteriorated by this point, and for all intents and purposes the inner line was the capital's main line of defence . Starting from the 1520s, proposals were made to strengthen the defences of this region, but the plan was disrupted by the local populace's resistance to the prospect of labour; only in the 1540s did work proceed in earnest . </P> <P> From 1544 to 1549, a defensive building program took place on a scale unprecedented in Chinese history . The project was led by Weng Wanda (翁 萬達; 1498--1552), the Supreme Commander of the Xuan--Da defence area (宣 大 總督), which was responsible for the Xuanfu, Datong, and Shanxi areas . Troops were re-deployed along the outer line, new walls and beacon towers were constructed, and fortifications were restored and extended along both lines . Firearms and artillery were first mounted on the walls and towers around this time, for both defence and signalling purposes . The project's completion was announced in the sixth month of 1548, but the walls were steadily augmented for a time after that . At its height, the Xuan--Da portion of the Great Wall totalled about 850 kilometres (530 miles) of wall, with some sections being doubled - up with two lines of wall, some tripled or even quadrupled . The outer frontier was now protected by a wall called the "outer border" (外邊, wàibiān) that extended 380 kilometres (240 mi) from the Yellow River's edge at the Piantou Pass (偏 頭 關) along the Inner Mongolia border with Shanxi into Hebei province; the "inner border" wall (內 邊, nèibiān) ran southeast from Piantou Pass for some 400 kilometres (250 mi), ending at the Pingxing Pass; a "river wall" (河 邊, hébiān) also ran from the Piantou Pass and followed the Yellow River southwards for about 70 kilometres (43 mi). </P>

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