The Roman Empire was the superpower of the ancient era, the Roman Empire dominated the western world for (a really long time). In 330 AD the Empire was effectively split into two parts for logistic reasons. Rome was the centre of the Western Roman Empire and Constantinople the centre of the Eastern, or Byzantine Empire.
The SCA takes its starting point for period as the time where Rome effectively ceased to be the centre of an Empire, around 600 CE, although its last official emperor, Romulus Augustus (or Augustulus, meaning the little, or insignificant, Augustus) was deposed, by the Germanic chieftain, Ordoacer, in 476. In the next century the Eastern Empire, under Justinian and his general Belisarius, invaded Italy and seized the city of Rome for a period.