Byzantine Architecture
The Hagia Sophia is renown for being one of the first masterpieces of Byzantine architecture. This style of architecture developed in the Byzantine Empire during the fifth century A.D. and is characterized by large, domed roofs that sit on top of square bases. Originally built as a way of remembering Roman architecture, the Byzantine architectural style went on to influence Medieval architecture throughout Europe and the Near East, as well as the Renaissance and Ottoman architectural styles that followed the Byzantine Empire’s collapse. It eventually gave way to Carolingian, Romanesque, and Gothic architecture in the West, while exerting a profound influence on early Islamic architecture in the East. The majority of structures that follow this style were built for religious purposes. Besides the large, domed roofs, Byzantine architecture is characterized by geometric complexity, large, round arches, and intricate glass mosaics decorating buildings’ interiors. In addition to stone, brick and plaster were used for the decoration of important public structures, while thin sheets of alabaster were placed over windows to softly illuminate the interior of a building. In order to place the large domes on top of square buildings, a constructive device known as a pendentive was used. Pendentives are shaped like triangular sections of a sphere and work by distributing the weight of the dome into four corners where the force can be received by piers. Overall, the Byzantine Empire was able to build off of previous architectural styles and incorporate new, innovative techniques of their own to develop a beautiful architectural style that is still admired to this day.
Sources:
https://en.wikipedia.org/wiki/Hagia_Sophia#Upper_Gallery
https://en.wikipedia.org/wiki/Byzantine_architecture
http://www.buffaloah.com/a/archsty/byz/