The Church of Saints Sergios and Bacchos was built by Justinian before he became emperor in 527. It has the form of a vaulted octagon with niches on the diagonal sides that is enveloped by a ambulatory with two storeys. Originally it formed a double church together with the basilica of St. Peter and Paul that disappeared already in the early middle Byzantine time. The church of Sergios and Bacchos still exists as a mosque. The similarity of its architectural carving to that of Hagia Sophia was the reason why the Ottomans gave it the name of "Small Hagia Sophia".

For more about this church see the following link: The Byzantine Monuments: THE MONASTERY OF THE STS. SERGIUS AND BACCHUS