Wary of the ever-present threat of invasion from Persians in the east and from Germans in the north and east, the remnant of the Roman Empire in the east established a distinct form of government that can react quickly to sudden threats. It was characterized by being tightly centralized wherein power totally lies in the hands of a highly revered emperor. This vision of society was then materially expressed in the ostentatious build up of a new capital that featured museums, libraries, artistic treasures and magnificent edifices made up of marble. The empire built a capital to portray wealth and power to intimidate its enemies.
Justinian was an important emperor who further developed the capitol Constantinople and more importantly established a lasting legal and social order by codifying Roman law into the Justinian Code. Through his brilliant general Belisarius, he was able to re-conquer what was Western Rome but was not able to retain control due to lack of resources. The Byzantine empire maintained its size and invincibility in those times.
The religion of Islam and its Arab adherents emerged in the 7th century and threatened religious conquest of Byzantine. They were able to conquer the eastern and southern territories of Syria, Palestine, Egypt, and north Africa and even reached and laid siege on Constantinople.
The city was able to repulse the attack and reinforced the Byzantine empires military resilience. It may not be able to expand further its borders but it solidified its hold on Greece and the Balkan region.
The empire's military strength and political stability allowed an economy to flourish undisturbed. Peasants were able to develop a robust agriculture that created surpluses for trade. Trade and banking was busy from all directions as the empire sits in the middle of the civilized world. Its Constantinople reached the apex of acclaim as the ultimate in cities in those times.