CITADELS (C Loa)
 
  In 257BC, King An Duong (Thuc Phan) built Co Loa or Loa Thanh (Snail-Form Citadel), located 15KM from Hanoi, in the province of Phuc Yen (presently Vinh Phu). Co Loa formed by 3 inter-twined mud-enclosures in labyrinthine fashion that looked like a snail-shell. The first enclosure measured 2X3KM, the second enclosure measured 6.5KM was positioned on higher ground, and the rectangular citadel formed the third enclosure. In the center was the royal palace, surrounded by quarters reserved for the guards.
The first betrayal of Viet Nam recorded in history was committed by My Chau, a royal princess who enticed by the charms of Trong Thuy, a northern prince. She told him the defense secret of the Snail-Form Citadel. Subsequently, the fortress fell into the hands of Ch'ao To (Trieu Da), Trong Thuy's father.
     
  Chinese-style Citadels
In 3BC, the Ch'in rulers of China deported massive numbers of ethnic Chinese into the south hoping to assimilate the Viet people. The Han who succeeded to the Ch'in actually imposed government control over Giao Chi. From this contact the Vietnamese learned military construction techniques from the Chinese and eventually put them to use in the defense of their territory.
In 43AD, when Chinese general Ma "the Conqueror of Waves" Yuan came south to repress the rebellion of the Trung sisters, he met with resistance coming from 65 fortresses or fortified residences. These constructions were built with primitive techniques, consisted of a huge square yard enclosed by mud-walls with cornered watchtowers. Entrance structures with holes covered by thatch through which small archers may be shot were on 4 sides of the citadel. Three little forts formed the outer defense line as alarm posts.
   

Hoa Lu
In 968, after defeating the Twelve Feudal Lords, Dinh Tien Hoang realized that the ancient capital was exposed and difficult to defense. He decided to retrench back to Hoa Lu, his native land and at the base of the cordillera.
Dinh Tien Hoang practiced the art of movement warfare. He believed it was illusory to build a solid or permanent citadel. Rather, Hoa Lu was conceived by him to be a temporary structure, permitting a holding action while his army could prepare to fight guerrilla-style in the bushes when confronted with huge and powerful Chinese army.
The walls of Hoa Lu fortress were made of huge brick blocks and wooden watchtowers in the distance. The royal palace located in the center of this defense system has been ruined, leaving no traces.
 
The Ho Citadel
In 1397, Ho Qui Ly built Tay Do ("Western Capital"), located in Tay Giai village, Quang Hoa subprefecture, Thanh Hoa province. In 1407, it was abandoned when Ho Qui Ly and his son, the ruling prince Ho Han Thuong, were taken prisoners by the Ming troops.
The citadel was built out of solid materials and according to a well studied plan. The enclosure was a square with 4 vaulted stone entrances on 500M length sides. In the center was Nhan Tho Cung, a royal palace. The citadel located behind a series of hills that served as defense enclosures and a river to the west served as a defense ditch. Thick ramparts were made out of cob and reinforced by a freestone wall that would shelter various constructions inside. The 7M long X 1.5M wide freestone blocks, weighing about 16 tons each, still standing which bear witness to an advanced construction.
   
  French-style Citadels
In 1790, Gia Long built Gia Dinh Thanh, the first French-style citadel, with the assistance of Olivier de Puymanel who was an architect of fortifications and military arts.
In 1802, Gia Long became Emperor. Two years later, he selected the ground for the Hue Citadel based on geomantic, natural and strategic considerations. In 1805, the construction started and continued under Minh Mang until its completion in 1831.
These quadrilateral or polygonal structures (5, 6 or 8 sides) were built in important provincial capitals of Viet Nam: Nam Dinh, Thai Nguyen, Thanh Hoa (shown), Dong Hoi, Quang Tri, Hung Yen, Ha Tinh and Saigon.
   
   

Chi D. Nguyen
References:
L'architecture Vietnamienne, Nguyen Quang Nhac