 Justinian also strengthened the borders of the Empire from Africa to the East through the construction of fortifications , and ensured Constantinople of its water supply through construction of underground cisterns ( see Basilica Cistern ) . To prevent floods from damaging the strategically important border town Dara , an advanced arch dam was built . During his reign the large Sangarius Bridge was built in Bithynia , securing a major military supply route to the east . Furthermore , Justinian restored cities damaged by earthquake or war and built a new city near his place of birth called Justiniana Prima , which was intended to replace Thessalonica as the political and religious centre of Illyricum . 
