Thanh Hoa tourism officials are hoping the ancient citadel built during the Ho dynasty (1400-07) that was recognised as a UNESCO World Cultural Heritage site in June will attract more holidaymakers to the area. The citadel, mentioned in Dai Viet Su Ky Toan Thu (The Complete Annals of the Great Viet), was built during the reign of King Tran Thuan Tong (1397) by Royal Mandarin Ho Quy Ly, who acceded to the throne in 1400. According to researcher Trinh Sinh, archaeologists were stunned by the huge building work. Thousands of huge rectangular stone blocks weighing between 10 and 20 tonnes were stacked seven high, and no mortar was used. The first two layers were below ground to support the structure.