 However, eventually, these internal weaknesses caused the Parthian Empire to collapse and the Sassanid Dynasty rose.Constantinople, situated on the Bosporus Straits at the mouth of the Black Sea, became the capital of the Roman Empire in 330 AD after Constantine the Great, the first Christian emperor, refounded the city of Byzantium.Although the city was called Constantinople until its fall, the Eastern Roman Empire became known by the classical name of Byzantium, and often the city was called by its old name as well.