Defensive wall, northern China. One of the largest building-construction projects ever carried out, it runs (with all its branches) about 4,500 mi (7,300 km) east to west from the Bo Hai (Gulf of Chihli) to a point deep in Central Asia. Large parts of the fortification date from the 7th to the 4th century BC. In the 3rd century BC the emperor Shihuangdi connected existing defensive walls into a single system fortified by watchtowers. These served both to guard the rampart and to communicate with the capital, Xianyang (near modern Xi'an) by signal — smoke by day and fire by night. Originally constructed partly of masonry and earth, it was faced with brick in its eastern portion. It was rebuilt in later times, especially in the 15th and 16th centuries. The basic wall is about 23 – 26 ft (7 – 8 m) high; at intervals towers rise above it to varying heights. It was designated a UNESCO World Heritage site in 1987.