Byzantine church design was a mixture of Greco-Roman design, with the basilica as its model. The basilica was different than a temple. It was a rectangular building and was constructed for civic purposes. Basilicas were buildings in which court took place, and people met with their lawyers, for example. Their purpose was to hold public gatherings, much like a town hall.
They had a central nave and aisles, and an apse at each end. There might be a sculpture of the Emperor, for instance, but they were pretty simple. Entrances were found, usually, in the long sides. After legalizing Christianity Constantine chose the basilica as a model for an early Christian church, and Old St. Peter’s was established based on this design. The classical design was altered to meet the needs of the Christian church, and became a cross-in-a-square or a cross-in-a-dome plan.
The Hagia Sophia was built with a cross-dome plan, and has influences from both the west and the east—from Greece and Rome, as well as from Asia. This church has been the star of the show since its beginning:
Hagia Sophia looms so large in the story of Byzantine architecture—and indeed in the history of all architecture—that it easily overshadows all the buildings in its near proximity, throwing them into obscurity…Of the extensive literature on the church, a large part is understandably devoted to structural problems (Mathews, pg. 88).
Justinian hired two designers named Anthemius and Isidore. They were called “mechanikoi” or masters of science and mechanics. Anthemius was a mathematician and a physicist, and Isidore was a professor of geometry and mechanics. It is unknown why they were chosen for the job of designing the Hagia Sophia, but they designed one of the most famous churches in the world. It is also a significant monument. The Hagia Sophia was the largest church in the world for almost a thousand years until the Seville Cathedral in Spain was completed in 1520.