A unique mosque housed underground in a crypt belonging to what was once The Castle of Galata. During the Byzantine era, entry to the Golden Horn was controlled from here with the Byzantines stretching a chain to deter invaders. The building was converted into a mosque in 1757. Inside, there are a series of columns supporting low vaulted ceilings to allow for private prayer space. The tombs of two Arab soldiers, killed in the first Arab siege of Constantinople in 714, can be found inside a glass case in the side chapel.