Answer:
Three correct answers:
Byzantium had a natural harbor.
Byzantium is located in one of the best natural harbors in the world, in a small peninsula in front of the strait of Bosphorus, which links the Black Sea with the Mediterranean Sea (via the Sea of Marmara and the Aegean Sea).
Rome had been destroyed by invaders.
Rome was sacked in 410 by an army of Visigoths lead by king Alaric. The sacking proved fatal for Rome, and the city would not regain its condition for over a thousand years after.
Byzantium provided greater access to trade routes.
Besides being a great port, Byzantium is also a link between Europe and the Middle East (and Asia), and the Mediterranean and the Black Seas. This means that Byzantium was at a crossroads, and had access to several trade routes.
