Ancient structure: Excavation holes at Co Loa Citadel. Three archaeological excavations conducted at the site since 2007 show that it's the oldest and largest earth structure of its kind in Southeast Asia. — Photo vietnamplus.vn

HA NOI (VNS) — Co Loa Citadel, 20km north of Ha Noi, appears to be the oldest such structure in Southeast Asia.

The finding comes after seven years of archaeological excavations by researchers from the Viet Nam Archaeology Institute, Thang Long Ha Noi Heritage Conservation Centre and University of Wisconsin-Madison in the US.

"The citadel shows ancient Vietnamese people's unique innovation during the long process of protecting the country from foreign invaders," researcher Trinh Hoang Hiep from the Viet Nam Archaeology Institute said at a recent workshop.

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. Various relics of the Bronze Age Dong Son culture can be found at the site, which was recognised as a special national relic site in 2012 by the Prime Minister.

Ancient history books in both China and Viet Nam mentioned the citadel, according to Hiep. However, the books mentioned the same events occurring in different places and at different times, confusing scholars.

The archaeological results provide concrete data about when the citadel was built.

Researchers also found that the three layers of walls were built using different techniques.

While the outer and middle walls were built in circle form, the inner layer was built in flat square form.

Based on historic materials gathered from excavations since 1970, researchers assumed that people of the late Dong Son civilisation (700-100BC) built the citadel.

"Before Co Loa was built, there was no other construction site in such a large area in the Hong (Red) River Delta," Hiep said. "In order to build Co Loa, a strong army might have been mobilised."

A large amount of brick tiles and stones was found at the site, which led to another hypothesis: could a tiled roof have been built above the citadel walls?

The research also proved that archaeologists were making advances in their methodology, according to chairman of the National Cultural Heritage Council Luu Tran Tieu.

"Scientists have started to approach sites in detail rather than just doing general research," he said.

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.

Co Loa is a very large site and is the dominant presence in the northern flood plain of the Red River Delta, so building it would have required a large amount of labour and resources.

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. — VNS