The city founded by Constantine in 330 CE on the strait between Europe and Asia that served as the capital of the Roman Empire, then the Byzantine Empire for over a millennium, before falling to the Ottoman Turks in 1453 and becoming Istanbul — a transformation that marked the end of the medieval world.
For over a thousand years, Constantinople was simply 'the City' — the largest, wealthiest, most fortified urban center in the Western world. When it fell in 1453, Europeans were so shaken that many historians treat the date as the end of the Middle Ages. Today it is Istanbul. The story of that transformation is the story of how the medieval world ended.