This was seen as a vindication of Yu Zijun's strategy of wall-building by the people of the border areas. By the mid-16th century, Yu's wall in the Ordos had seen expansion into an extensive defence system. It contained two defence lines: Yu's wall, called the "great border" (大邊, "dàbiān"), and a "secondary border" (二邊, "èrbiān") built by Yang Yiqing (14541530) behind it. 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 ). 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. However, wall construction continued regardless of court politics during this time. The Ordos walls underwent extension, elaboration, and repair well into the 16th century. Brick and stone started to replace tamped earth as the wall building material, because they offered better protection and durability. This change in material gave rise to a number of necessary accommodations with regard to logistics, and inevitably a drastic increase in costs. Instead of being able to draw on local resources, building projects now required brick-kilns, quarries, and transportation routes to deliver bricks to the work site. Also, masons had to be hired since the local peasantry proved inadequate for the level of sophistication that brick constructions required. Work that originally could be done by one man in a month with earth now required 100 men to do in stone. 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 (abbreviated as "Xuan–Da") 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.