As per the Legend, Lord Krishna killed the demon Kamsa (his maternal uncle) in the wrestling area and made Ugrasena (his Maternal Grandfather) as the king of Mathura. Knowing Kamsa's death, his father-in-law, demon Jarasandha started attacking Mathura along with this friend Kalyavana. They together invaded on Mathura 17 times. Keeping the safety of their citizens, Lord Krishna then decided to move the capital from Mathura to Dwarka.
Lord Krishna invited Viswakarma (divine architect) to build the capital on the coast of Saurashtra. Viswakarma then built the Golden Capital City which was gifted by Samurdadeva. It is said that the beautiful Dwarka city was built on 12 Yojanas.
Dwarka into the Sea
As soon as Lord Krishna end his Avatar, Yadavas started quarrelling among themselves and lost the control. Balarama Deva too left his Avatar on seeing this and no heads to take care. Sea raised high and Submerged the Golden city Dwarka.