 Complete the following phrase:  Byzantine emperor Justinian I rebuilt Singidunum from scratch in 535, restoring the fortress and city to its former military importance. Singidunum became an important border stronghold, however, Justinian rebuilt only the area within the former legions camp. The city saw a brief peaceful period of about fifty years, but was then sacked with the arrival of the Avars in 584, though the Byzantines recaptured it in 596. The city remained under the constant attacks of the Avars and their allies at the time, Slavs. During 