What dynasty built walls ?
Sometime between 1190 to 1196 , during Zhangzong 's reign , the high official Zhang Wangong ( 張萬公 ) and the Censorate recommended that work on the wall be indefinitely suspended due to a recent drought , noting : " What has been begun is already being flattened by sandstorms , and bullying the people into defence works will simply exhaust them . "