In 1009, Ly Dynasty was initially established with the enthroning of Ly Cong Uan King. In July, 1010, the royal decree called Chieu Doi Do was promulgated to change the capital to Dai La Citadel from Hoa Lu (Ninh Binh province). After transferring the capital city, the King commanded to build The Citadel of Thang Long and it was latterly completed in the early 1011.
Due to its extremely important roles, the Citadel of Thang Long was surrounded by three separate fortresses. The outer fort was Kinh Thanh (Imperial City), which most of citizens lived. It was surrounded by the Hong, Kim Nguu and To Lich rivers, Kinh Thanh played an essential role as a dyke system for the capital city. The second fort (the middle ring) was Hoang Thanh (Imperial Citadel), which the royal court, offices and residence of mandarins were located. The smallest and most important was Tu Cam Thanh (Forbidden City) which the king, queens and concubines lived in seclusion.
The Citadel of Thang Long was repaired and had many new works in Tran Dynasty and expanded in Le So Dynasty. The Citadel of Thang Long was repaired and expended its mass area in Tran and Le So Dynasties under many renovations and reparations. Tn dynasties of Mac and Le Trung Hung (from 1516 to 1788), the Citadel of Thang Long was destroyed many times. In 1789, Quang Trung King transferred the capital city to Phu Xuan, and the Citadel of Thang Long only acted as Bac Thanh (the northern defensive fortification). In the Nguyen Dynasty, the remainders of the Imperial Citadel of Thang Long were transferred to Phu Xuan for building new citadel. The Kinh Thien Palace and Hau Lau were retained to be accommodations for Kings Nguyen during their business trips to the Bac Thanh. In 1805, Gia Long king ordered the demolition of walls surrounding the Imperial Citadel of Thang Long and requested the building of a new one that was smaller citadel called Ha Noi Citadel with architectural style of Vauban (France). In 1831, in a big administrative reform, Minh Mang king changed name of the Citadel of Thang Long to Ha Noi Province. When French colonists occupied all Indochina, Ha Noi has choosen as the capital of French Indochina Union and the Citadel of Thang Long was destroyed to build military camp. The Ha Noi Citadel has become the headquarters of the Ministry of Defense since the Vietnamese army took the control of the capital city in 1954. The first value of the central sector of Imperial Citadel of Thang Long shows that it is nearly a book displaying over 10 century- history of Thang Long – Ha Noi from Dai La Citadel in Pre-Thang Long period to nowadays.
Nowaday, the Imperial Citadel of Thang Long in Hanoi covers a large area of 20 ha consisting archaeological area at 18 Hoang Dieu Street and relics in Hanoi Citadel such as: Doan Mon, Bac Mon, Hau Lau, dragon house, house D67, Ha Noi flag tower and stone dragons in Kinh Thien Palace. These are 4 streets surrounded the relics such as: Nguyen Tri Phuong Street in the east and Hoang Dieu Street in the west, Dien Bien Phu Street in the south, Phan Dinh Phung Street in the north.