which was the most powerful empire of the time?
The Roman Empire
Middle Ages
The Roman Empire reached its greatest territorial extent during the 2nd century AD; the following two centuries witnessed the slow decline of Roman control over its outlying territories. 
what is the time period of the middle ages? 5th to the late 15th century
wiki:25614220
Middle Ages [SEP] Later Roman Empire
The Roman Empire reached its greatest territorial extent during the 2nd century AD; the following two centuries witnessed the slow decline of Roman control over its outlying territories. Economic issues, including inflation, and external pressure on the frontiers combined to create the Crisis of the Third Century, with emperors coming to the throne only to be rapidly replaced by new usurpers. Military expenses increased steadily during the 3rd century, mainly in response to the war with the Sasanian Empire, which revived in the middle of the 3rd century. The army doubled in size, and cavalry and smaller units replaced the Roman legion as the main tactical unit.