How did Constantinople start?

Answer 1

It didn't so much 'rise' to power. It was simply part of the old Roman Empire and when that fell in the late 5th century the city, and the East (and a bit of Italy) were all that really remained of the Roman Empire. It 'inherited' its power, if you will. After the last Roman Emperor died in 476, the East was all that was left of the Roman Empire. Though realistically Constantinople had been a power-base since its inception (in the early 4th century) and many people consider it to have been 'the new Rome' though Constantine himself never actually said this.

An argument could possibly be made that Constantinople's prosperity was induced by Constantine's interest in the region and thus it rose to power steadily but it was really the fall of the West which shows it for the true Mediterranean power it was.

Answer 2

Constantinople was founded by the Roman Emperor Constantine I in 324 AD on the site of an already-existing city, Byzantium, which in turn was established by the ancient Greeks some 1,000 years before.

Answer 3

Emperor Constantine the Great moved the capital of the Roman Empire from Rome to Constantinople around 330 AD. He felt that Rome was an unsatisfactory capital. Rome was too far from the frontiers. Constantinople provided easy trade and military access to the Mediterranean, Black Sea, Danube River, Dnieper River, the land route to Turkestan and India.