According to source C The Great Wall of China the dynasty who first stared building the wall was the Qin's dynasty they didn't finished it then the Han dynasty continued building the wall the benefits did not outweigh the cost. Pretty sure not because Mongols still passed over the wall that cost china a lot of deaths. Plus the wall cost lot of time effort and Money all for nothing. During the short ten-years period of Qin wall-building, there was heavy use of peasant laborers, who worked seven-days work weeks with little food. During the Han period peasants farm families were forced to move north and west both
