After inheriting the cause of nation building of 18 Hung Vuong dynasties, An Duong Vuong Thuc Phan defeated fifty thousand Qin invaders; renamed the country Van Lang to Au Lac and moved the capital from the mountains of Nghia Linh, Phong Chau to Phong Khe area, also known as Ke Chu area, ie Co Loa, Dong Anh, Hanoi today.

An Duong Vuong immediately started to build the citadel, but it fell down every day and night, but it didn't finish. The king then sent the mandarins to set up a forum to pray to the gods and ask for help from the gods. On the seventh day of the third month of that year, the king suddenly saw an old man with gray hair and beard, leaning on a bamboo cane, walking slowly from the east to the front gate of the city, tilting his neck and lamenting, "Building this city knows how to build this city. never be done!". Overjoyed, King An Duong brought the old man into the palace, respectfully asked: "I have spent a lot of effort building this citadel, but it didn't work, what's the excuse?" The old man leisurely replied: "There will be a Thanh envoy. Jiang came with the king to build a successful new exam." After saying this, the old man said his goodbyes and left.

The next morning, a large tortoise emerged from the water, claiming to be the Thanh Giang porcelain, and told An Duong Vuong that if he wanted to build peace, he had to eliminate all the monsters that often harass him. Sure enough, after the Golden Turtle helped the king eliminate the demon, it only took about half a month for the city to be built. The citadel was built in the shape of a spiral, more than a thousand feet wide, so it was called Thanh Oc or Loa Thanh. The Golden Turtle stayed for three years and then left. At parting. An Duong Vuong thanked and said: "Thanks to God, the city has been built. Now, if there is an enemy from outside, what will you use to fight it?" The Golden Tortoise took off a claw and gave it to An Duong Vuong, telling him to use it as a crossbow. Come, just aim and shoot, you won't have to worry about it anymore. With that, the Golden Turtle returned to the East Sea. The king sent a talented general, Cao Lo, to create a large crossbow, using the Golden Tortoise's claws as a paddle. Gold turtle.


 
Not long after, Trieu Da brought troops to invade Au Lac. An Duong Vuong took out a magic crossbow to shoot, and each shot killed thousands of people. They panicked and ran back to Buffalo Mountain, held out for a few days and then withdrew back to the country. Au Lac people joyfully celebrate the glorious victory of the talented king.

Seeing that he could not swallow Au Lac by attacking, Trieu Da thought of another sinister plot. Praying for his son, Trong Thuy, to propose to My Chau, An Duong Vuong's beloved daughter. Without a doubt, the king willingly married and even allowed Trong Thuy to stay in-law in Loa Thanh.

According to his father's instructions, Trong Thuy secretly paid attention to everywhere and set out to discover the secret of the light-hearted Me Chau god crossbow, but truly loved her husband, so she brought Trong Thuy to the place where Trong's crossbow was hidden. Thuy made a paddle that looked exactly like the real thing and then swapped it and replaced it with the Golden Turtle's claw. After finishing work, Trong Thuy said to his wife: "The love between husband and wife cannot be forgotten, the meaning of mother and father cannot be abandoned. I will return to visit my father. she, what is the mark?". Mi Chau innocently replied: "I have a goose feather shirt that I usually wear, when I meet a change, wherever I go, I will sprinkle goose feathers at the crossroads to make a sign to find each other".

When Trong Thuy returned home, Trieu Da immediately sent troops to fight Au Lac. Hearing the news that tens of thousands of enemy troops had flooded, hoping to have a magical crossbow, An Duong Vuong still sat leisurely playing chess and laughed, "Isn't Da afraid of the magic crossbow?". Quan Da approached the gate of the city, the king ordered the magic crossbow to shoot, but it didn't work anymore.

The father and son had to get on their horses, aiming to the South to run, but wherever they ran, the enemy followed the goose feathers and chased them there. Coming to the coast, An Duong Vuong along the way shouted loudly: "Where is Thanh Giang porcelain, come and save me!". Immediately, the Golden Turtle appeared, pointed at My Chau and told An Duong Vuong: "The one sitting behind is the enemy!". Understanding the reason, An Duong Vuong got angry, drew his sword to cut My Chau, just at that moment, Trong Thuy also arrived. He rushed in to block An Duong Vuong's cruel sword on behalf of his beloved wife. Suddenly, the water parted, the Golden Turtle welcomed An Duong Vuong into the sea. Mi Chau burst into tears and ran after her father, but angry waves of white foam stopped her steps. She collapsed next to her husband's body, sobbing.