Cổ Loa

Cổ Loa Citadel (Vietnamese: Thành Cổ Loa) is an important fortified settlement and archaeological site in present-day Hanoi's Dong Anh district, about 16 kilometers (10 mi) northeast of Hanoi city center. Various relics of the Bronze Age Phung Nguyen culture and Dong Son culture have been found in Cổ Loa, although it was later established as the capital of Âu Lạc Kingdom during the 3rd century BC (about 257 BCE). Further construction was added during the later dynasties. Cổ Loa remained an important political center of the Vietnamese people until the 10th century. The name "Cổ Loa" is derived from the Sino-Vietnamese , meaning "old spiral", reflecting its multi-layered structure of earthworks, moats and ditches.

According to folklore, Thục Phán defeats the last of the Hung kings in 257 BCE and founded the kingdom of Âu Lạc, choosing the site of Cổ Loa as his capital. Given its relatively large size, Cổ Loa maintained its dominant presence in the northern floodplain of the Red River Delta and would have required a large amount of labour and resources to construct.

The mythical story goes that when the fortress was being built, all the work done during the day was mysteriously destroyed at night. The king made a sacrifice to the gods and in one night, a golden turtle appeared to him in a dream and told him the fortress was built on the turtle's carapace. The king was instructed to build the city in a new location, that of present-day Cổ Loa. The king did so, and the city was soon finished.

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 the thousands. However, one of the Qin dynasty general, Zhao Tuo, took advantage of the decline of the Qin and created his own kingdom north of Âu Lạc called Nanyue. He tried to conquer his southern neighbour but was defeated. Instead, he married his son to the daughter of the Thục Phán. When the son was in Co Loa, he discovered the magic turtle's claw and stole it. His father then proceeded to invade Âu Lạc and easily defeated it.

...
Wikipedia