Byzantium's Strategic Location

The Bosporus is the narrow strait between Europe and Asia, running from the Black Sea down to the Sea of Marmara. The northern side in particular was quite sharp, which meant that the horn was protected from the wind. This allowed the Byzantine navy to always have a safe port from which to move out to attack any hostile fleet. His small distance also meant that the Byzantines could fairly easily hop over to Asia in no time at all. With enemy navies struggling to make an impact, the Byzantine ships could fetch supplies from other parts of the empire to resupply the city. And with access to land to the north through the Black Sea or just across to Asia