After the fall of the Western Roman Empire in 476, the Eastern Roman Empire of which Constantinople was the capital, became the Byzantine Empire. Thus, Istanbul was transformed from a "Roman City" to a somewhat Orthodox one, with an eastern accent.
The mid-sixth century marked the beginning of an enlightened age for the Byzantine Empire, and thus, for Istanbul. In contrast with his predecessor, who was not even literate, Emperor Justinian I. was an educated and religious man. During his reign the city prospered as an Orthodox Christian capital. St. Sofia’s Church was reconstructed in this period.
However, the plague of 543 killed almost half of the city’s popullation, disaster followed disaster. Fortunately, the infrastructure built by Emperor Justinian I. had made the city fairly resilient agains manner of catastrophes and wars. The late 7th and 8th centuries became years of siege. In the 7th century, Istanbul was attacked by both Persian and Avars. Later, in the 8th century, Hungarian and Muslim Arabs besieged the city. Russian and Hungarian forces, in the 9th century, also tried to conquer this desirable metropolis.
Meanwhile, sectarian conflicts among Christians had become violent, fuelled by the politics of the Emperor who took a decisive position in the matters. The pro/anti - iconography which divided the population bore a tremendous impact, not only on the city, but on the entire empire and on Christian theology in particular. Istanbul’s thriving era was eclipsed by Latin occupation.
In 1204 the city was conquered by the Crusaders and was looted mercilessly. The largest city of the Middle Ages, with a population of nearly 500.000 laid impowerished, and in ruins.