The incursions of various barbarian tribes made imperative the building of fortifications at strategic points. This was a particular concern of the emperor Justinian, as is evident from the work of the historian Prokopios -"On Buildings"-, where 46 fortresses are mentioned in Macedonia alone. All along the frontier earlier Roman fortifications were repaired or reinforced and new ones raised.
Built of undressed stones and mortar or, in the more careful constructions, of broad zones of undressed stones alternating with narrow zones of brick (walls of Thessalonike), the fortifications were as a rule square or rectangular in plan and strengthened at intervals by towers. The wall of Thessalonike, like that of Constantinople, had two enceintes and surrounded the city from the seaward side too.