Why did Byzantium manage to repel the attack of the barbarians in the 6th century?

Byzantine Empire, a state that arose in the 4th century. at the collapse of the Roman Empire
in its eastern part and existed until the middle of the 15th century.
The capital of V. was Constantinople.
B. 4th – 5th centuries. was a centralized military-bureaucratic monarchy.
All power was concentrated in the hands of the emperor (basileus).
The Senate was the advisory body under the emperor.

Since the 70s. 4 c. not only the foreign policy, but also the internal political situation of Byzantium
largely determined the relationship of the empire with the barbarians.
In July 400 barbarians almost took possession of Constantinople, and only thanks to the intervention
in the struggle of broad strata of the townspeople, they were expelled from the city.
By the end of the 4th century. with the increase in the number of mercenaries and federates, the Byzantine army was barbarized;
temporarily, at the expense of barbarian settlements, small free land tenure and colonies expanded.
While the Western Roman Empire, which was experiencing a deep crisis, fell under the blows of the barbarians.