The labor force was made up of three groups:
soldiers, common people, and criminals.
Soldiers made up the main construction group. This was the case with the building of the wall of Northern Qi (550-557). Also, during the Qin Dynasty (221BC - 206BC), 300,000 soldiers were redirected to build the wall after General Meng Tian conquered the Huns. Still, it took nine years to finish the work. In order to take care of the soldiers and make them feel settled, the court even arranged for widows to marry them.
Millions of common people were called up to build the wall. Northern Qi's wall, starting from Xiakou to Hengzhou, used 1,800,000 laborers. A section of Sui's (581 - 618) wall in Inner Mongolia required more than 1,000,000 men to build. Besides the first 300,000 soldiers, Qin's wall required another 500,000 common people to complete the work.
Criminals were forced into labor as punishment. During the day, they were ordered to take turns patrolling the wall, while at night, building and extending the wall. They were required to serve four years of hard labor according to laws at that time.