The city of Hué comprised three interlocking enclosures:
The Kinh Thành citadel, the outermost of the three, housed administrative buildings,
The Hoàng Thành imperial city, housing royal palaces and pilgrimage sites,
Tu Cam Thanh Forbidden Purple City, to house the royal residences.

Added to this was the Tran Binh Dai defensive structure in the north-west corner of the capital, whose function was to monitor movements on the river. Another fortress, Tran Hai Thanh (coastal bastion), was added some time after the first to protect the capital against assaults from the sea.

The plans took two years to draw up, from 1803 to 1805, and construction was not completed until 1832. Over 30,000 workers and soldiers were involved in the construction, which involved filling in the Perfume River and excavating new ditches and canals. The fortress itself was, for the first time in Asia, modelled on European models in the style of Vauban.

Here’s the map of the Imperial Citadel and Forbidden City (in the dark blue square)

The Imperial Citadel buildings include :
1. Ngo Mon ------------------------------------2. Thai Dich Lake
3. Trung Dao Bridge ---------------------------4. Dai Trieu Court
5. Thai Hoa Palace ---------------------------6. Dai Cung Mon
7. Ta vu, Huu vu ------------------------------8. Can Chanh Palace
8a. Vo Hien Palace ---------------------------8b. Van Minh Palace
9a. Trinh Minh Palace -----------------------9b. Quang Minh Palace
10. Can Thanh Palace ----------------------11. Khon Thai Palace
11a. Thuan Huy Institute --------------------11b. Sanatorium
12. Kien Trung Palace ------------------------13. Thai Binh Lau
14. Ngu Uyen Garden -------------------------15. Co Ha Garden
16. Internal Affairs Office -------17. Trieu Mieu
18. Thai Mieu ----------------------------------19. Truong Sanh Palace
20. Dien Tho Palace --------------------------21. Phung Tien Palace
22. Hung Mieu -------------------------------23. Le Mieu
24. Nine Dinh ---------------------------------25. Hien Lam Cac
26. Hien Nhon gate -------------------------27. Hoa Binh Gate
28. Chuong Duc gate -----------------------29. Ngu Tien office
30. Luc Vien -----------------------------------31. Minh Than Palace.
32. Duyet Thi Duong ------------------------33. Thuong Thien
34. Royal Hospital.
– – – Dans la zone de la Cité Interdite (dans le carré bleu foncé) se trouvent ces structures :
6. Dai Cung Mon
7. Ta vu, Huu vu
8. Palais Can Chanh
8a. Palais Vo Hien
8b. Palais Van Minh
9a. Palais Trinh Minh 9
b. Palais Quang Minh
10. Palais Can Thanh
11. Palais Khon Thai
11a. Thuan Huy Institute
11b. Sanatorium
12. Palais Kien Trung
13. Thai Binh Lau
14. Jardin Ngu Uyen
29. Bureau Ngu Tien
30. Luc Vien
31. Palais Minh Than
32. Duyet Thi Duong
33. Thuong Thien
34. Ngu Y Vien

Detailed urban planning was entrusted to Nguyễn Van Yen, commander of a military unit specializing in citadel construction.

The architectural ensemble was damaged during military operations in 1885, 1947, 1960 and 1975 ....

In Alphonse Guérin’s collection, only the Ngo Mon Gate is photographed; to see the other monuments of the Imperial Citadel or the Forbidden City, we have to go through the views by Dieulefils, Sesmaisons...