The fortified city of Constantinople was founded on the site of Byzantium on the Bosporos in AD 324 by Constantine, Roman Emperor from 324 to 337. The city was dedicated in 330.
(see The Founding of Constantinople.)

Constantinople (now Istanbul) was capital of the Eastern Roman Empire and its location formed as a gateway city between the Black Sea and the Mediterranean.

Its location meant that the city lay on both the Silk Roads and the Spice Roads which made it an important trading post of the late Roman Empire. The city grew in wealth and power, and as the focus of the Roman Empire shifted eastward, the city became more and more important, and eventually the emperor of Constantinople became the ruler of the entire Roman Empire.

Aqueduct of Valens in Constantinople
image: Wikimedia