Co Loa Citadel, 20km north of Hanoi, appears to be the oldest such structure in Southeast Asia. The citadel was built under the reign of King An Duong Vuong (around the 2nd century BC). Its name "Co Loa" is derived from the Sino-Vietnamese for "old spiral" as the structure is built in a spiral shape. 

According to folklore, Thuc Phan (An Duong Vuong) defeated the last of the Hung kings in 257BC and founded the kingdom of Au Lac, choosing the site of Co Loa as his capital. 

The story goes that when the city was built, all work done during the day was destroyed at night. The king made a sacrifice and that night a golden turtle came to him in a dream and told him that he was building the city on the turtle's carapace. The turtle instructed him to build the city in a new location, that of present-day Co Loa. The king did so, and the city was soon finished. 

The city was in the shape of a conch shell and had nine walls, each protected by a moat. The moats were part of a series of streams and lakes that exist to this day and provided Co Loa with protection and navigation. 

Out of gratefulness to the king, the magic turtle gave the king a claw that he could use as a trigger on his crossbow. When used, it multiplied its force by thousands of times. 

However, one of the Qin dynasty leaders, Zhao Tuo, took advantage of the decline of the Qin and created his own kingdom north of Au Lac. He tried to conquer his southern neighbour but was defeated. Instead, he married his son to the daughter of the Thuc king. When the son was in Co Loa, he discovered the magic turtle's claw and stole it. His father invaded Au Lac and easily defeated Co Loa. 

Stories of the Thuc king's demise vary. Some say he committed suicide by jumping in the ocean. Some say he was borne off to sea by the magic turtle and never seen again. Some say he was told of his daughter's betrayal by the magic turtle and killed her before disappearing with the turtle.