1. Justinian was a Byzantine emperor who rebuilt the city of Constantinople and sought to reconquer former Roman territories after the fall of the Western Roman Empire. 2. He commissioned the construction of new fortifications, aqueducts, and most notably Hagia Sophia, one of the greatest churches ever built. 3. Justinian also had Roman laws collected and codified into the Justinian Code, which served as the basis of Byzantine law for over 900 years.