A monumental building project which created a wall running (with all its branches) about 4,000 miles (6,400 km) west to east from Bohai Bay to a point deep in central Asia, the Tarim Basin. Parts of the vast fortification date from the 4th century BC. In 214 BC, the first emperor of a united China (Shih Huang-ti of the Qin dynasty) connected a number of existing defensive walls into a single system fortified by watchtowers, which served both to guard the rampart and to communicate with the capital, Hsien-yang, by signal - smoke by day and fire by night. The enemy against whom the Great Wall was built were the Hsiung-nu, the nomadic tribes of the northern steppes. The wall was originally made of masonry and rammed earth and was faced with brick on its eastern portion. It was substantially rebuilt in later times, especially in the 15th and 16th centuries. The basic wall is generally about 30 feet high, and the towers are about 40 feet high.