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.