The city of Constantinople is extremely important in the history of Christianity, arguably as important as Rome. It was the centre of Christianity in the East and it was here that Christians resisted the Muslim advance.
Constantinople had held out against incredible odds for a long time. But it could not last forever. This interesting video gives a helpful summary of the events surrounding the fall of Constantinople. This event was influential not just in political but in religious spheres as well.
For more information on Constantinople, I recommend the History of Byzantium Podcast.