The Great Wall of China was built over centuries by China’s emperors to protect their territory. Today, it stretches for thousands of miles along China’s historic northern border. 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. Emperor after emperor strengthened and extended the wall, often intending to keep out the northern invaders. In some places, the wall was constructed of brick. Elsewhere, quarried granite or even marble blocks were used. The wall was continuously brought up to date as building techniques advanced.

Source: National Geographic