How did the Emperor of Qin try to protect his empire from invaders?

The Emperor of Qin had taken control over all of China by 221 B.C. To stop invading forces, he ordered a wall to be built. It was called 'the 10,000 Li Long Wall' (a Li = roughly 3/10 mile). It later became known as the Great Wall, but not the Great Wall we are familiar with. That was built by later rulers over the remnants of Qin's Great Wall. It took ten years and some 300,000 workers to build Qin's wall. The bodies of dead workers were buried in the wall.