Great Wall of China

Located in northern China.. It was built as a defensive wall 20-15 feet high and 15-25 feet thick, with towers at intervals. It was contructed using labor of over 300,000 men (mainly prisoners). Without its branches and other secondary sections, the wall extends for around 4,160 miles (6,700 km), often tracing the crestlines of hills and mountains as it snakes across the Chinese countryside. In the 3rd century BCE, Shih huang-ti, the first emperor of a united China, connected a number of existing defensive walls into a single system; large parts of the fortification date from the 7th through the 4th century BCE.