When the Byzantine Empire had split, it had moved its capital to Constantinople. The main reason they had moved it into Constantinople was the fact that it was in the middle of the Black and Aegean sea which would give them access to trade routes. In addition, Constantinople was one of the richest cities at the time, and it also had a border between the Christians and the Muslim faith. With that being said, European soldiers had to come into Constantinople in order to fight the holy war. In my own opinion, I think the byzantines switched their capital in order to gain these advantages and trade routes.
By 527 CE, Justinian was crowed as emperor of the Byzantine empire. During his time he had expanded, and had reclaimed some land back in the west. Furthermore, Justinian and his wife, Theodora had stopped peasants from taking control of the Byzantines. Without the help of Theodora, the Byzantine empire may had collapsed early on. As you can see, Theodora and Justinian might've not been the best emperor, but they certainly saved the Byzantine empire.
