Throughout the fifth century, the Byzantines build Theodosian Walls (of the same quality of Constantinople's) around most of their major cities. Jerusalem, Alexandria, Antioch, Thessalonica, Corinth, Dyrrachium, Serdica, Trebizond, Aleppo, and Damascus have full Theodosian walls by the dawn of the sixth century.
Justinian won't be able to go on his reconquista due to the treasury being depleted and the imperial army is definitely going to suffer in the short term.
No reconquista means (probably) no Heraklios, but will the Byzantines be able to survive the Persian crisis (assuming it even happens) by having their major cities better fortified?
Of course, civil wars will be a lot harder to put down. What are the short and long term effects on the Empire?