The Imperial Citadel of Thang Long was the cultural complex which comprises Thang Long Imperial City during the dynasties of King Ly, King Tran and King Le and Hanoi Citadel during the Nguyen Dynasty. Through the ups and downs of Vietnamese history, the remains of Imperial Citadel of Thang Long are roughly where the Hanoi Citadel lies today.

The history has proved that the royal palaces and almost structures in the Imperial Citadel of Thang Long were seriously damaged in the late of 19^th century. At present, the ruins of Thang Long Citadel are still being excavated.

Generally, the ancient Hanoi Citadel had three encircling walls. Tu Cam Thanh (The Purple Forbidden Citadel) was house of the King and his royal family. It is located within the inner wall inside Hanoi Citadel. There were different names of the citadel through various dynasties, such as: Cung Thanh in the Ly Dynasty, Long Phuong in the Dynasty of Tran and Cam Thanh (the Forbidden Citadel) on the Le Dynasty. Only one gate which functioned as the connection of the Purple Forbidden Citadel and Hoang Thanh (the Royal Citadel) was called Doan Mon Gate.

Inside the middle wall was the Royal Citadel where royal audiences were taken place. It was also a place of offices and residences of mandarins. Time has taken its tolls to many gates linking the Royal Citadel with Kinh Thanh (the outmost wall); only Bac Mon (the Northern Gate) in present-day Phan Dinh Phung street has survived.

Lastly, the earthen outmost wall was the inhabitation of common people, called Kinh Thanh. In the past, Kinh Thanh used to have many opened gates. During the Le dynasty, 16 gates were recorded, but by the Nguyen dynasty, only 12 of them had survived. Until the early of 20^th century, there were five Gates remaining as Cho Dua, Dong Mac, Cau Den, Cau Giay and Quan Chuong, of which Quan Chuong Gate has been preserved to date and the others now only live in the memory of Hanoians.

Nowadays, tourists can visit the vestiges of the ancient earthen wall at Dai La, Hoang Hoa Tham, and La Thanh roads. On the anniversary of 1000 years Thang Long – Hanoi, the central sector of the Imperial Citadel of Thang Long (Hanoi Citadel) was proudly recognized as one of the World Heritage Sites by UNESCO on July 31^st 2010 in Brazil.