Numerous buttresses have been added throughout the centuries. The flying buttresses to the west of the building, although thought to have been constructed by the Crusaders upon their visit to Constantinople, are actually built during the Byzantine era. This shows that the Romans had prior knowledge of flying buttresses which can also be seen at in Greece, at the Rotunda of Galerius in Thessaloniki and at the monastery of Hosios Loukas in Boeotia, and in Italy at the octagonal basilica of San Vitale in Ravenna.[1] Other buttresses were constructed during the Ottoman times under the guidance of the architect Sinan.