The Great Wall of China is the world's longest man-made structure, stretching approximately 1,500 miles (2,400 km) along China's northern and northwestern borders. Erected to prevent invasions by northern nomads(遊牧民), it is an amalgamation of many walls built in ancient times and connected by later empires. Averaging a height of 25 feet (7.6 m) and a width of 15 to 30 feet (4.6 - 9.1 m) at its base, the wall is fortified(防備を強化する、要塞化する)by watchtowers placed at regular intervals.