"For millennia, Chinese leaders instituted wall-building projects to protect the land from northern, nomadic invaders. One surviving section of such an ancient wall, in the Shandong province, is made of hard-packed soil called "rammed earth" and is estimated to be 2,500 years old. For centuries during the Warring States Period, before China was unified into one nation, such walls defended the borders. 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. 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." (National Geographic: The Great Wall of China)