The Great Wall was built and rebuilt, and extended during a period of 1600 years, beginning in the second century BC. It was built for two reasons, one practical the other psychological. The practical matter was that the nomadic tribes to the north were better horsemen than the Chinese were, and there was a need to keep them out. The wall was not originally as massive and solid as it became over the centuries and it was not entirely effectiev anyway-- the Mongols under Genghis Khan broke through it. The psychological one was that the emperor who started the project was immodest and wanted to build a grand monument to himself. The number of casualties involved in building it numbered in the many thousands, and lots of them were buried in the wall itself. Slaves may have been used, but slaves were defined as people who were in debt, so the definition and concept of slavery was not one of permanence as it was in the US. Asian cultures still keep to this definition of slavery in many ways, as you can find families in rural Myanmar and other SE Asian countries that sell a child to erase debt.