The ancient Hanoi Citadel had three encircling walls. Within the inner wall was Tu Cam Thanh (The Purple Forbidden Citadel) where the King and his family lived. Thị Citadel has been called differently through various dynasties: Cung Thanh in the Ly dynasty, Long Phuong in the Tran, and Cam Thanh (The Forbidden Citadel) in the Le. Doan Mon functioned as the only gate connecting the Purple Forbidden Citadel with Hoang Thanh (the Royal Citadel).

The Royal Citadel lied inside the middle wall where royal audiences were held. It also housed 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.

Common people inhabited the earthen outmost wall, Kinh Thanh. 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. Of the five gates remaining until the early 20^th century (Cho Dua, Dong Mac, Cau Den, Cau Giay ang Quan Chuong), only Quan Chuong gate has been preserved to date. The remaining four gates now live in the memory of Hanoians. The vestiges of the ancient earthen wall can be seen at Dai La, Hoang Hoa Tham, and La Thanh roads.

In preparation for the great events to take place in Thang Long – Hanoi, much has been done to restore the Thang Long (Hanoi Citadel).

(Source: vietnamtourism.com)