The old name of the Kalenderhane Mosque, which lies at the foot of the Bozdogan (Valens) Aqueduct is not known with certainty. It is thought that it could be the Akaleptos Monastery, first referred to in the llth century. Immediately after the conquest this building was turned into a mosque by a pious foundation set up by Mehmet the Conqueror himself and allocated to the Kalender Dervishes. The interior, which is shaped like a cross with arms of equal length, is covered by a dome. The coloured marble facings on the wall next to the altar niche indicate that its interior was richly decorated. A very interesting find was encountered in a cell to the right (south) of the altar niche during investigations and restoration work being carried out by an American group in the Kalenderhane Mosque. This was an inscription in Latin written in Gothic letters containing the name of Saint Francis of Assisi. with coloured marmor and with friezes in the form of reliefs.
Apart from the fact that it was used as a Catholic church between 1204 and 1261,this is also the oldest reference to the Church of Saint Francis,(a saint who lived from 1181 to 1226), which is one of the most important churches in the European Christian world. The Kalenderhane church was converted to a convent and personally allocated to the Kalender Dervishes by Mehmet II., the Conqueror. The convent, in turn, was converted to a mosque by Babussaade Agasi Maktul Besir Aga during the 18th Century. only keeps their memory alive in its name. The building was demolished by a fire in the 19th Century and had to be rebuilt in 1854. In 1930, a lightning struck the mianrette and caused it to collapse. The mosque ceased to be used as a place for prayer and was eventually abandoned. The Istanbul University carried out joint studies and restoration work with Harvard University at the building between the years 1966-1975. Restoration finished in 1968 and the mosque opened once again for worship in the same year. The walls of the mosque are a mixture of stone and brick. A dome is located on the roof of the building. The interior is decorated.