 While the Yongle Emperor had staged five major offensives north of the Great Wall against the Mongols and the Oirats , the constant threat of Oirat incursions prompted the Ming authorities to fortify the Great Wall from the late 15th century to the 16th century ; nevertheless , John Fairbank notes that " it proved to be a futile military gesture but vividly expressed China 's siege mentality . " Yet the Great Wall was not meant to be a purely defensive fortification ; its towers functioned rather as a series of lit beacons and signalling stations to allow rapid warning to friendly units of advancing enemy troops . 
