While various states in China had built walls since the 7th century BC, the first unified Great Wall was commissioned by Qin Shi Huang, the first Emperor of a unified China. He ordered the connection of existing northern fortifications to protect his new empire from the Xiongnu nomadic tribes. This massive engineering project required the labor of hundreds of thousands of soldiers and convicts, many of whom died during construction. Most of what remains today, however, was built during the Ming Dynasty (1368–1644) using stone and brick rather than the rammed earth used by the Qin. The Great Wall is not a single continuous line but a series of walls, towers, and gates stretching thousands of miles. It served not only as a military defense but also as a means of border control and trade regulation along the Silk Road, symbolizing the power and isolationism of imperial China.