Rome was sacked in the year 410 by the Visigoths and again in 455 by the Vandals. Its last emperor was deposed in the year 476. These events helped propel the rest of the world into a new age–a dark age–while the Roman Empire in the East continued to survive and even thrive at some points for another millenium.
In the five years between 532 and 537, the emperor Justinian had the Church of Hagia Sophia built. This church was a powerful symbol of the new empire that was formed. Justinian took the idea of a unified empire seriously, and he put a lot of effort into retaking land that had been lost when the western half collapsed under wave after wave of invasion. Unfortunately, this was also a period of Islamic expansion that threatened to unravel everything Justinian strived to achieve.
During the period between 610 and 641, the modern-day areas of Syria, Palestine, and Egypt were taken by Muslim invaders. By the 690s they had taken Byzantine in North Africa. From 717 through 718, Constantinople itself was under siege. The invasion failed, but it was an example that the Byzantine Empire was not invincible–even if it was a force with which to be reckoned.
Over the next centuries, the Roman church continued to gain power and influence. The Byzantine Empire was ruled by the Macedonian dynasty, founded by Basil I. This dynasty ruled from 867 until 1056 and ushered in the Macedonian Renaissance, which led to a transformational period for Christian artwork and scholarship in general.
The decline of this eastern empire began around 1050. This was the age of the Crusades, which tore through surrounding lands between 1096 and 1291. The Byzantines were smashed by Seljuk Turks in 1071, and then Constantinople was lost to Crusaders with the Fourth Crusade in 1204. It took nearly sixty years until Constantinople could be liberated, an event that occurred in 1261.
Constantinople finally fell for the last time in 1453 after a massive force of Ottoman Turks ruined the city with a barrage from heavy artillery and infantry. This would change everything. The city would be forced to live under new Islamic rule with unfamiliar Muslim laws. Christians were not allowed to own weapons, although they were not barred from practicing. The city was renamed Istanbul, and the rest is history!