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)

Photo taken on Aug. 23, 2019 shows ancient bricks and stone bases in the Imperial Citadel of Thang Long in Vietnam's capital Hanoi. 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)

Photo taken on Aug. 21, 2019 shows an architectural pattern in the Imperial Citadel of Thang Long in Vietnam's capital Hanoi. 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)

Photo taken on Aug. 21, 2019 shows two pigeons resting on an ancient wall of Thang Long Imperial Citadel in Vietnam's capital Hanoi. 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)

A woman wearing Vietnamese traditional costume walks 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)

Photo taken on Aug. 21, 2019 shows a view of the Imperial Citadel of Thang Long in Vietnam's capital Hanoi. 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)

Photo taken on Aug. 23, 2019 shows Doan Mon (South Gate) of the Imperial Citadel of Thang Long in Vietnam's capital Hanoi. 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)

Local people walk near an ancient wall of 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)

Photo taken on Aug. 23, 2019 shows an interior view of Doan Mon (South Gate) of the Imperial Citadel of Thang Long in Vietnam's capital Hanoi. 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)

Photo taken on Aug. 21, 2019 shows the Flag Tower in the distance in the Imperial Citadel of Thang Long, Vietnam's capital Hanoi. 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)