Between 5000 BC to 2000 BC, Neolithic cultures flourished in parts of China using stone tools, pottery, and simple agriculture. During the Han Dynasty, China was unified and the population increased under stability, while foreign trade and improved industry helped society. The Great Wall was built to keep out northern Mongolians and was strategically planned along mountainous terrain, though workers were treated cruelly. Ancient Chinese seafaring peoples were believed to be the world's first "boat people" with astonishing seamanship sailing thousands of miles using sails and other devices on ships symbolizing the emperor's greatness.