This area has a 2,200 year history of being encircled by walls. The first was built in 194 BC during the Han dynasty (202 BC – 220 AD). The defense protected the former Changle Palace. It was the world’s largest palace (1,200 acres) before being destroyed. In the Tang dynasty (618-907), a 16 mile wall was erected around their capital city of Chang’an. This citadel served several dynasties until Zhu Yuanzhang founded the Ming dynasty in 1368. At the start of his 30 year reign, Hongwu Emperor ordered a new, stronger wall be constructed on the crumbling foundations. The project required eight years of labor. Subsequent enhancements and restorations occurred during the 16th, 18th and 20th centuries. The ancient fortification survived the Revolution of 1911 and Japanese air raids during World War II. In 1961, the historic landmark became a UNESCO World Heritage Site. This is the northeast watch tower, one of four at the corners of the Xi’an City Wall. There are 24 smaller, two-level watch towers positioned along the ramparts.