This is perhaps the most ancient citadel in Vietnam, built by King An Duong Vuong between 214 and 208 BC to make it the capital of the Au Lac State (the name of Vietnam at that time). The citadel has the structure of an oyster shell with 3 turns: the exterior, the middle and the interior. Legends said that one night the white cocks badly damaged the citadel.

The god of river knew the damage and mysteriously interfered by sending a holy tortoise to protect the citadel building.
When the construction was completed, a magic bow was given to king An Duong Vuong to defeat the northern invaders from China.
The Chinese King Trieu Da knew the story and planned to have his son, Prince Trong Thuy, marry Princess My Chau the daughter of King An Duong. Trong Thuy made use of the opportunity and stole this magic weapon.
War happened shortly after that, the Vietnamese king lost. He, together with his daughter committed suicide. Commemorating the king and the princess with their good deeds, his people constructed a temple to worship them which still remains intact up to now.
Visitors will also find the remains of the citadel and many vestiges here.