Traditionally, the Roman Empire fell in 476 AD. At this time it consisted of Eastern and Western sections. The Western, ruled from Rome, dissolved partly as a result of bringing in barbarian peoples, as Germanic mercenaries with no allegiance gradually destroyed imperial power.
The wealthier Eastern, or Byzantine, Empire continued and Constantine renamed Byzantium Constantinople in 324 AD, making it the capital of the Empire. In 325 AD, with the 1st Nicaean Council, Constantine increased his power over the Christian Church.
Justinian, Emperor from 527 to 557 AD, together with his brilliant and influential wife, Theodora, restored the glory of the Empire and codified its laws in a form that remains the basis of most European legal systems.