After surviving over 1000 years, the remnants of the Eastern Roman Empire finally began to crumble. The city of Constantinople had been renamed to Byzantium. This was the centre of the great Byzantine Empire, which was a centre of learning, and lay between Western Europe and Islam both culturally and geographically for much of the Middle Ages. By 1453, the Empire was reduced to the area surrounding Byzantium: that was all.
The Ottomans were a Turkish tribe which had been launching invasions of Byzantine territory for centuries. By 1453 they had the gates of Byzantium surrounded, and the city finally fell. The city was renamed once again, his time to Istanbul. This is still the city's name, and it's the capital of modern day Turkey.
After the invasion, many of the academics, scholars and monks who had called Byzantium home fled to parts of Western Europe. They took with them centuries of learning: art and literature from Ancient Roman and Greek times were again introduced to Western Europe.