[2][3] Justinian Prima was originally designed to become the capital of the prefecture of Illyricum, but for reasons likely related with its status near the Roman frontiers of the 6th century CE, Thessaloniki was preferred.

[8] The city was to become capital of Illyricum, but Thessaloniki was preferred and Justinian Prima received jurisdiction over the territories of the Diocese of Dacia.

The city planning combined classical and Christian elements: thermae, a levantine agorai, and streets with colonnades.

And close by this place he built a very notable city which he named Justiniana Prima, thus paying a debt of gratitude to the home that fostered him.

For to enumerate the churches is not easy, and it is impossible to tell in words of the lodgings for magistrates, the great stoas, the fine marketplaces, the fountains, the streets, the baths, the shops.

Invading Avars coming from north of the Danube may be one factor, missing political interest in the town after the time of Justinian may be another.