Fenâri Îsâ Mosque was made of two former Eastern Orthodox churches. In 908, the Byzantine admiral Constantine Lips inaugurated a nunnery on the site. The church was built on the remains of another shrine from the 6th century, and used the tombstones of an ancient Roman cemetery. Relics of Saint Irene were stored here.
After the Latin invasion and the restoration of the Byzantine Empire, between 1286 and 1304, Empress Theodora, widow of Emperor Michael VIII Palaiologos (r. 1259–1282), erected another church dedicated to St. John the Baptist south of the first church.
In 1497–1498, shortly after the Fall of Constantinople, the south church was converted into a mescit (a small mosque). The edifice burned down in 1633, was restored in 1636 by Grand Vizier Bayram Pasha, who upgraded the building to cami ('mosque') and converted the north church into a tekke (a dervish lodge). The building burned once more in 1918, and was abandoned. During excavations performed in 1929, twenty-two sarcophagi have been found. The complex has been thoroughly restored between the 1950s and 1960s by the Byzantine Institute of America, and since then serves again as a mosque.
As a whole, this complex represents a notable example of middle and late Byzantine Architecture in Istanbul.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.