The style of architecture developed in the Byzantine Empire developed after the 5th century; massive domes with square bases and round arches and spires and much use of mosaics WordNet 3.0 & Open English WordNet

Byzantine architecture is the architecture of the Byzantine Empire, or Eastern Roman Empire, usually dated from 330 AD, when Constantine the Great established a new Roman capital in Byzantium, which became Constantinople, until the fall of the Byzantine Empire in 1453. Wikipedia