The figure at the top of the Column of Trajan is actually St. Peter. It was previously a statue of Trajan himself, but the statue went missing in the Middle Ages. It was replaced with the current bronze statue of St. Peter by Pope Sixtus V in 1587.

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Throughout the history of Christian art, Jesus Christ has been depicted in a variety of roles. During the Byzantine era, depictions of Christ as the Pantocrator, or "ruler of the world," were common. Images of the Pantocrator appear at the top or center of Byzantine hierarchical compositions, and are often flanked by angels.

The deep carving on the Hagia Sophia's pillars and in other decoration makes them appear hollow and too delicate to support the cathedral's weight. The close-set windows lining its golden dome's base create the illusion that the dome is floating. Together they suggest that the whole building is supported by some divine power rather than by its own structure. 

The Hagia Sophia was originally built as an early Greek Orthodox church. It is difficult to consider the architecture as of a particular time period or style as it has been changed and influenced so many times. There are, however clear remnants of Greek Classical style as well as the early hints of what will become the Romanesque church style. 

Image adapted from http://www.publicdomainpictures.net/hledej.php?hleda=hagia+sophia+5.

The church is a combination of central and Basilica plan. It has all of the standard additions of those styles, including the nave and apse, as almost every church of this type does. The narthex is identified as the porch-like addition opposite the apse. This church does not have an atrium, however, which would look like a long open plan that sits before the church door.

Image adapted from http://www.publicdomainpictures.net/hledej.php?hleda=hagia+sophia+5.

The Hagia Sophia has all the standard additions of the Basilica-plan church, including the apse opposite the door, a nave in the center, and aisles on each side. It lacks the cross-like arms of later Basilica churches. It also has a domed top and a square-ish base, which are both central-plan additions. It is unique in this way among its contemporaries.

Image adapted from http://www.publicdomainpictures.net/hledej.php?hleda=hagia+sophia+5.

The current layout of the Hagia Sophia was begun by Justinian II in 532. It was ordered after his predecessors tried and failed to build a monumental building in that area. As Justinian was largely regarded as the first Byzantine emperor, the Hagia Sophia became a seminal work for Byzantine and later Eastern Orthodoxy churches.

Image adapted from http://www.publicdomainpictures.net/hledej.php?hleda=hagia+sophia+5.

Isidore of Miletus, along with his companion the mathematician Anthemius of Tralles, was commissioned by Justinian I to create the Hagia Sophia. Anthemius died shortly before construction could begin, so Isidore continued alone. He was considered the father of the current structure, minus the Islamic artistic additions.

Image adapted from http://www.publicdomainpictures.net/hledej.php?hleda=hagia+sophia+5.

The towers are called minarets. They are an Islamic addition to the church, added when it became a mosque in the fifteenth century after the Turks sacked Constantinople. These towers are not universal, but are common in Islamic mosque architecture. 

Image adapted from http://www.publicdomainpictures.net/hledej.php?hleda=hagia+sophia+5.

The Hagia Sophia was built just after the birth of the Roman Christian tradition, and came well before the advent of Renaissance oil paintings in churches. It was also built in the Middle East, and was influenced by the artistic traditions of the area. It therefore likely has mosaics on the inside, which are the cornerstone of Byzantine church art.

Image adapted from http://www.publicdomainpictures.net/hledej.php?hleda=hagia+sophia+5.