The old citadel is located in a large campus, bordering Hoang Dieu ward in the west, Nguyen Tri Phuong road in the east, Tran Phu street in the south and Phan Dinh Phung street in the north, belonging to Ba Dinh district.
In 1010, king Ly Thai To moved the capital from Hoa Lu to Thang Long, the citadel was built and enlarged in post-dynasties. Thang Long citadel was built in three circles. The forbidden city or dragon phoenix city in Ly - Tran dynasty is in the center of the citadel. The royal city is in the middle and encircles a business zone eastwards. The outermost part is La Thanh or Dai La Thanh. In 1805, the Nguyen dynasty built Hanoi citadel and retained the forbidden city as a place for the king to stay while visiting the North. In the French colonial period, the French retained this area as the army head quarters after destroying Hanoi citadel.
Five ancient architectural works remained after Hanoi citadel was destroyed are the flag tower, Doan Mon gate, Kinh Thien palace, Hau Lau and Bac Mon gate.
Doan Mon is the only work that is fairly undamaged. It has 2 storeys; the upper one has 8 roofs and the lower one has 5 doors. Above the southern door, there is a line of "Doan Mon".
In 1886, Kinh Thien palace was destroyed to build the headquarter of French Artillery. Now, there are only stone corridors with two stone handrails imitating two dragons in the middle. The southern staircase of Kinh Thien palace consists of 10 steps and the northern one has 7 steps formed by big rocks that are joined together.
Hau Lau, or building of princess, was built from bricks. The lower structure is in the shape of a box which has three roofs, the upper one is a three - storeys architectural work with two roofs.
Bac Mon gate is in trapezium shape with two triangular sides. There are two stone steles on the gate: the first one is in the north of the gate and was engraved with Chinese letters "Chinh Bac Mon", the second one is on the right side and was engraved with April 25, 1882 and two cannon bullet traces as the signs of the war between Vietnam and France.
The five vestiges are not large – scale architectural work but they are evidences of a political centre, a capital of the country throughout nearly thousand years.
In the south of Ha Noi citadel, there is a part of the first university of Vietnam – Van Mieu – Quoc Tu Giam that was built in the early eleventh century in the Ly dynasty.