To solve this problem, Shi Huangdi used prisoners as workers to build new roads and canals. These workers built more than 4,000 miles (6,437 km) of roads. This linked even the distant parts of the empire to the center of the Qin government at Xianyang (shee»AHN»yahng).
Shi Huangdi found that protecting his large empire was not easy. To the north of the empire lived large tribes of fierce warriors who rode horses. In earlier times, the people of the northern kingdoms of China had built walls of rammed earth to protect their borders from these people. But these walls did not keep the invaders away for long. Shi Huangdi ordered his workers to link together the existing walls. Using this forced labor, the Qin created a long, single wall a Great Wall.
The Great Wall stood 30 feet (9 m) high, with 40-foot (12-m) towers. The long wall twisted and turned through mountains, valleys, marshes, and deserts for more than 1,500 miles (2,414 km). Yet in spite of its size, the Great Wall was built in just seven years.
The Great Wall not only kept invaders out of China, it warned people when invasions were taking place. Soldiers on the Great Wall communicated with each other by using signals. Smoke was used as a signal during the day, while fire was used as a signal at night. Signals would travel from tower to tower along the Great Wall, until they reached the Qin capital.