Kalenderhane Mosque in Istanbul’s Vefa district is one of the city’s most interesting buildings in which Byzantine and Ottoman history can be read within the same structure. Standing close to the Valens Aqueduct, it remains an active mosque today, although its origins lie in the Byzantine period. Its original identity was debated for many years, but twentieth-century archaeological research made it widely accepted that the building was probably the church dedicated to the Theotokos Kyriotissa — the “Mother of God Enthroned.”

The present building is the final stage of a long sequence of construction on the same site. Archaeological research has shown that earlier structures existed here from Late Antiquity onward. The main building visible today, developed over the remains of earlier churches, is generally dated to the Middle Byzantine period. Kalenderhane is therefore not simply a surviving Byzantine church, but evidence of the way Constantinople repeatedly reused and rebuilt the same sacred place.

The plan is based on an important Byzantine type: the domed Greek cross. Four barrel-vaulted arms extend from the central domed space, while corner compartments and the sanctuary to the east complete the composition. Alternating courses of stone and brick on the exterior are characteristic of Middle Byzantine architecture in Constantinople. The building appears heavy and enclosed from outside, but the central space beneath the dome creates a much stronger sense of verticality once inside.

One of the most remarkable episodes in Kalenderhane’s history followed the capture of Constantinople during the Fourth Crusade in 1204. During the Latin occupation, the building was used by Catholics. Frescoes depicting scenes from the life of Saint Francis, discovered during twentieth-century restoration and now preserved in the Istanbul Archaeological Museums, are exceptional documents of this period and among the earliest known representations of Saint Francis in Constantinople.

Other important paintings were also revealed during the same work. These include scenes connected with the life of the Virgin and, most notably, a mosaic of the Presentation of Christ in the Temple. Since much of the interior decoration of Byzantine churches in Istanbul has disappeared over the centuries, these fragments suggest that Kalenderhane once possessed a far richer interior than its present restrained appearance might imply.

After the Ottoman conquest of Constantinople in 1453, a new phase began. The former church was assigned for a time to Kalenderi dervishes, from whom the name “Kalenderhane” derives. It was later converted into a mosque, with a mihrab, minbar and other elements required for Islamic worship. Because the eastward axis of the Byzantine church does not correspond exactly with the direction of Mecca, the mihrab was placed at an angle to the original architectural axis. This subtle shift remains one of the clearest physical signs of the building’s two religious lives.

Kalenderhane suffered fires, earthquakes and repeated repairs in later centuries and eventually fell into poor condition. Extensive archaeological investigation and restoration from the late 1960s through the 1970s, led by Cecil L. Striker and Doğan Kuban, not only stabilized the building but also revealed the historical layers hidden beneath and within it. The identification with Theotokos Kyriotissa was one of the major results of this research.

Photographically, what interests me at Kalenderhane is that these layers are not always visible at first glance. Behind today’s relatively austere stone and brick surfaces lie earlier Byzantine churches, the frescoes of the Latin period, the Kalenderi dervishes and centuries of Muslim worship. The dome, vaults and massive geometry preserve the Byzantine character, while the mihrab and minbar show the Ottoman and continuing Islamic life of the building.

What makes Kalenderhane valuable is not simply the survival of an approximately thousand-year-old monument, but the way the same place has repeatedly been reinterpreted and kept alive. Its journey from a Byzantine church in Constantinople to a Latin Catholic place of worship, then a dervish lodge and finally one of Istanbul’s living mosques is a small but powerful summary of the changing history of the city.