From Byzantine Studies by Paspates (1877)
Sekbanbaşı Mosque was a church located around 100 meters north of the Aqueduct of Valens probably dating to the Middle Byzantine era. Its identity is unknown. It seems to have continued to function as a church after the conquest of Constantinople in 1453. It was converted into a mosque in the late 15th century by the Sekbanbaşı İbrahim Ağa. After being severely damaged by the fire in 1918, the building was demolished to widen Atatürk Boulevard in 1943. However before the completion of the boulevard, its remains were surveyed in the early 1950s.
This small church-mosque (10 x 9 meters) had recessed-brick masonry that suggests it dates to the Middle Byzantine period. This variation of the cross-in-store type was rare in Constantinople. The naos did not have nine full bays; instead, the tripartite sanctuary replaced the three eastern ones. A distinctive feature of this church was the form of the three apses of the bema, which were rounded on the exterior, similar to the Church of St. John Prodromos in Trullo.
Plan by Schneider
Architecture and Ritual in the Churches of Constantinople: Ninth to Fifteenth Centuries by V. Marinis
Converted Byzantine Churches in Istanbul: Their Transformation Into Mosques and Masjids by S. Kirimtayif
Bildlexikon zur Topographie Istanbuls: Byzantion, Konstantinupolis, Istanbul by Wolfgang Müller-Wiener
Sekbanbaşı Mescidi (Byzantium 1200)
Sekbanbaşı Mescidi (NYU Byzantine Churches of Istanbul)