The Great Wall of China was built over centuries by China’s emperors to protect their territory. For millennia, Chinese leaders instituted wall-building projects to protect the land from northern, nomadic invaders. Around 220 B.C.E., Qin Shi Huang, also called the First Emperor, united China. He masterminded the process of uniting the existing walls into one. At that time, rammed earth and wood made up most of the wall. Emperor after emperor strengthened and extended the wall, often with the aim of keeping out the northern invaders.