What caused the decline of cities in the early Middle Ages and their revival from the X-XI centuries?

Due to the fact that during the early Middle Ages, most of the ancient ancient states waged war with the barbarian tribes that settled in the central and eastern part of Europe, after the Great Migration of Nations, the Roman Empire, as a commercial and cultural center, fell into decay.
Most of the funds and provisions went to the needs of the army, and the cities were constantly attacked by the barbarians.
As a result, people left large cities en masse.
Over the centuries, the situation has stabilized, yields have increased significantly, and work practices have improved, which has led to the revival of once magnificent cities.