The Citadel is Hue’s main attraction, having served as the primary residential estate of the royal family and hosting the main buildings of state. Built by Emperor Gia Long in 1804 on auspicious ground (determined by royalappointed geomancers), the Citadel is a colossal structure that sprawls out over the vast majority of the city centre. The squareshaped foundation is surrounded by 10km long walls that are two metres wide. Facing the river, the 37 metre-high flag tower is the highest in Vietnam. Beside the tower are the impressive Nine Holy Cannons.