The central sector of Imperial Citadel of Thang Long – Ha Noi covers area of 18.395ha, includes archaeological area at 18 Hoang Dieu Street and relics in Ha Noi Citadel such as: Ha Noi Flag Tower, Doan Mon, Kinh Thien Palace, Building D67, Hau Lau, Bac Mon, Forbidden City wall and eight gates from the Nguyen Dynasty. These relics are located in Ba Dinh District and surrounded by Phan Dinh Phung Street in the north; Bac Son Street and National Assembly Building in the south; Hoang Dieu, Doc Lap streets and National Building in the west; Dien Bien Phu Street in the southwest and Nguyen Tri Phuong Street in the east.
In 1009, Ly Cong Uan was enthroned, founded Ly Dynasty. In July, 1010, the king promulgated Chieu Doi Do (the royal decree) to change the capital city from Hoa Lu (Ninh Binh) to Dai La Citadel. After transferring the capital city, Ly Cong Uan had Citadel of Thang Long built and the citadel construction was finished in early 1011.
The ancient Citadel of Thang Long was encircled by three incorporated forts. The outer fort was Kinh Thanh (Imperial City), where the general public lived. Surrounded by the Hong, To Lich and Kim Nguu rivers, Kinh Thanh acted as a dyke system for the capital city. The second fort (the middle ring) was Hoang Thanh (Imperial Citadel), where the royal court, offices and residence of mandarins were located. The smallest and most inner enclosure was Tu Cam Thanh (Forbidden City) where 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. From 1516 to 1788 in dynasties of Mac and Le Trung Hung, the Citadel of Thang Long was destroyed many times. In early 1789, King Quang Trung transferred the capital city to Phu Xuan, the Citadel of Thang Long only acted as Bac Thanh (the northern defensive fortification).
In Nguyen Dynasty, the remainders of the Imperial Citadel of Thang Long were transferred to Phu Xuan for building new citadel. Only Kinh Thien Palace and Hau Lau were retained to be accommodations for Kings Nguyen during their business trips to the Bac Thanh.
In 1805, King Gia Long ordered the demolition of walls surrounding the Imperial Citadel of Thang Long and requested the building of a new, smaller citadel called Ha Noi Citadel with architectural style of Vauban (France). In 1831, King Minh Mang changed name of the Citadel of Thang Long to Ha Noi Province in a big administrative reform. When French colonists occupied all Indochina, they chose Ha Noi as the capital of French Indochina Union and the Ha Noi Citadel was destroyed to build military camp for French colonists. Since the Vietnamese army took the control of the capital city in 1954, the Ha Noi Citadel has become the headquarters of the Ministry of Defense. The first value of the central sector of Imperial Citadel of Thang Long – Ha Noi 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.