Shaṭpura means “six cities”. These were six fortified cities built under the earth by the six lakh asuras who survived the war of Tripurāsura. They lay along the Āvarta river, and Brahmā’s boon kept the devas from killing their people. Bhagavān Kṛshṇa destroyed them with the Sudarśana cakra.

Origin

After the Tripurāsura war, six lakh asuras who had survived did tapas (austerity) for a thousand years. They faced the sun and lived on air alone. In this way they pleased Brahmā. They asked him for a vara (boon) of vengeance against Rudra. Brahmā turned them toward another boon, immunity from the devas.

His boon had a condition. As long as they honoured brāhmaṇas and walked in righteousness, neither Mahādeva nor any other deva could kill them. If they turned against brāhmaṇas, death would come to them through Vishṇu. The Hari Vaṃśa Purāṇa tells this in the Vishṇu Parva, Chapter 84.

Location

The six cities were underground, below the surface of the earth along the Āvarta river, in the region near the Sahya mountain. The brāhmaṇa Brahmadhatta performed his Aśvamedha on the banks of this same river. The Vishṇu Parva, Chapter 85, tells this.

Bilvodeakeśvara and the Sudarśana

According to the Vishṇu Parva, Chapter 86, the Bilvodeakeśvara temple of Bhagavān Śiva stood near this place. Before the battle, Bhagavān Kṛshṇa and his army bathed in the divine Ānarta river and had darśana of Bilvodeakeśvara Śaṃkara. Bhagavān Śiva himself appeared at this temple. He told Bhagavān Kṛshṇa to use the Sudarśana against Nikuṃbha, the chief of the Shaṭpura asuras. Nikuṃbha was the last one to fall.

After the battle

Nikuṃbha was destroyed and Brahmadhatta completed his Aśvamedha. Then the land of Shaṭpura was given to Brahmadhatta. A great festival was held. The Yādavas and the ṛshis who had gathered there celebrated the return of order, and then went back to Dvārakā. This is told in the Vishṇu Parva, Chapter 87.

Sources

Hari Vaṃśa Purāṇa, Vishṇu Parva, Chapters 84–87 and 92