 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.