Two visitors walk in the Imperial Citadel of Thang Long in Vietnam's capital Hanoi, Aug. 23, 2019. The Imperial Citadel of Thang Long was built in the 11th century. Situated at the heart of Hanoi, the central sector of the Imperial Citadel of Thang Long was inscribed on the World Heritage List by the UNESCO in 2010. (Xinhua/Wang Di)