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 during this period.
However, the plague of 543 killed almost half of the city's population. Disaster followed disaster. Fortunately, the infrastructure built by Emperor Justinian I. had made the city fairly resilient against all manner of catastrophes and wars. The late 7th and 8th centuries became years of siege. In the 7th century, Istanbul was attacked by both Persians 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 split 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, lay impoverished, and in ruins.

THE LATIN INVASION

Istanbul first became familiar with the Crusaders in 1096. The Emperor Alexius rejoiced at the coming of the first crusaders, hoping to regain lands lost in Malazgirt. The agreement was for the Byzantines to support the crusaders and occupy lands taken tram the Moslems. The Crusaders didn't go along with the plan and founded the Eastern Catholic Kingdom in Jordan in 1099. The Crusaders were despised by the residents of Istanbul who openly showed their disfavour. Meanwhile, the Crusades continued and the fourth Crusade ended in the invasion and dividing up of Istanbul.
During that period there was a great deal of dissension as to the succession to the throne. The Crusaders, realising their opportunity, entered the Golden Horn with the aid of the Venetian. The attack began on 9 April and the city fell on 13 April. For three days, in an unprecedented example of barbarism, Istanbul was looted and the inhabitants murdered. Many important structures, including Haghia Sofia were damaged and books hundreds of years oid were burned. Important Byzantine works of art were taken to Europe. The looting became routine and the Crusaders joined with the Venetian to divide Byzantium among themselves, founding the Eastern Catholic Empire.
After this, Istanbul grew smaller and poorer. The wealthy and royal and many of the populace fled to Iznik (Nikia). The Eastern Catholic Empire only managed to be sovereign in Istanbul and its environs. Iznik (Nikia), Trabzon (Trebizand) and Epiros in Greece formed a Byzance alliance and surrounded the Eastern Catholic Empire m 1254. Istanbul became even more impoverished, so much so that the Emperor Baudoin II had to resort to using the wooden sections of the palace as fuel to provide heat. Finally, the Palatlogos noble family regained Istanbul, thus ending the Eastern Catholic period.

THE SECOND BYZANTINE PERIOD

The second period of the Byzantine Empire starts after the Palailogos Kingdom took Istanbul from the Latins in 1261. During this period. Istanbul would not be able to regain its previous importance and individuality.
During this unfortunate period, being subjected to a merciless pillage by the Latins, Istanbul lost many of its historical treasures, as well as its importance for international trade. The decline of Istanbul lasted until the end of this period. Istanbul became a farm city surrounded by fortresses. It lost its entire commercial superiority to Galata (Sycae). Galata became the center of trade leaving Istanbul behind.
Nevertheless, there was one positive improvement: during the Second Byzantine Period, the fight among religious factions calmed down. Istanbul became the center of Orthodox Christianity during this period.
Byzantine art excelled to its apex during this period. The mosaics on the wall of Kariye (St. Savior-in-Chora) Church is considered as the peak of Byzantine art.
During this period, Istanbul was in the center of a gradually shrinking circle surrounded by Ottomans who were conquering Byzantine land continuously In 1373 Istanbul started paying the Ottomans a tribute tax. In 1393 Sultan Yıldırım Bayezid and in 1422 Sultan Murad V besieged Istanbul, but they failed. Since Orhan Gazı, the Anatolian side of the Bosphorus was under the control of Ottomans. In the 15th century most of the Thrace, with the exception of several insignificant towns, were under the control of Ottomans.
Thus, the Byzantine emperors of the 15th century were frequently forced to ask help from Catholic Rome. But, in exchange for help, the Papacy demanded the unification of the Catholic and Orthodox Churches under the authority of Rome. In 1452, the Byzantines were compelled to submit to this condition. The demand to celebrate this unification in Haghia Sophia Church, in the center of Istanbul, caused bitter reactions and harsh protests.
With the fall of Constantinople in May 1453, the Byzantine Empire became history. This was the starting point of a new and a brighter period for Istanbul. ( s2 )