Modern Chinese and international research using new methods like drones systematically studies the Great wall of China. Its visibility from space is a legend, but its size actually ranks as world's largest monument. It never was a single wall, but a system of several segments, repeatedly altered over time, in successive attempts to prevent invasions from raider tribes, like the Xiongnu, adapted to the terrain, with an elaborate system of garrison towers, advanced tower posts and communication system. Building and manning it proved extremely costly. It failed to prevent the Mongol armies of Genghis Khan annexing China, his great successor Kublai Khan built a wall system to fit his larger empire. After the Ming dynasty finally expelled the Mongols, the 'primitive' walls were replaced by the final stone version, using a brick type solidified with rice starch. While the unprecedented mobilization of means for construction and garrisons crippled the treasury, it proved useless against the ...