The Fort of Galata (καστελλίου τών Γαλάτων), located near a small cape on the southeastern shore of Galata, was an important feature of Constantinople’s defenses, because it protected the northern side of the legendary chain protecting the Golden Horn. Now functioning as a mosque (Yeraltı Camii “Underground Mosque”), it is a multi-layered structure that provides a continuous link from the Byzantine and Ottoman periods to the present. During the Byzantine era, it was first a typical defensive structure and later a fortified arsenal under the Genoese, while it served many functions during the Ottoman era, including an arsenal, granary, customhouse, and mosque.

Throughout much of its history, it was positioned along the shores of Galata near the beginning of the Golden Horn, though it is now about 50 meters inland. The surviving substructure surviving as a mosque consists of a series of piers and groin vaults that first functioned as the undercroft of the fort (kastellion). It has a rectangular plan, roughly measuring 50 x 40 meters, with 42 visible piers (though it originally had as many as 63 piers). Its walls are around 2.25 meters thick, except for its northeastern side, which is less than 2 meters thick. Its current height of the vault arches is around 270-280 meters, but its original floor might lower than its current level.

The Fort of Galata was first mentioned during the Second Siege of Constantinople (717-718) when Leo III the Isaurian (717-741) blocked off the Golden Horn with a chain and equipped his navy with Greek fire. The fort served as a point of attachment for this chain that stretched across the mouth of the Golden Horn on enormous logs to the Tower of Kentenarion near the Acropolis in Constantinople. It is unclear when the fort itself was built, but it possibly dates to the reign of Tiberius III (698-705). At some point, at least by the 10th century it had at least one tower as well. Nikephoros II Phokas (963-969) made use of the chain during naval raids by the Rus’ in 969.

The Fort of Galata played a key role in the Fourth Crusade and the conquest of Constantinople in 1204. The Crusader naval assault, led by the Venetians, attacked the fort in July 1203 rather than the ships defending the Golden Horn behind the chain. Once the chain was broken, the Venetians quickly overcame what remained of Byzantine naval forces in the Golden Horn – forever ending of glory of the Byzantine navy and its dromons. Controlling the Golden Horn also made it easier for the Crusaders to sack Constantinople in April 1204. Michael VIII Palaiologos (1259-1282) failed in his attempt to take the fort in 1260 from the Latins, though he would take control of the fort the following year when he reconquered Constantinople.

In 1261, the Empire of Nicaea and the Republic of Genoa made an alliance at Nymphaeum to retake Constantinople from the Latin Empire. Despite strained relations in the aftermath of the reconquest of Constantinople, the Genoese were granted land in Galata in 1267. Before Galata came into the possession of the Genoese, its fortifications were demolished, except for the Fort of Galata, which would remain in Byzantine hands. It is unclear exactly when the Genoese began to build the walls of Galata, though evidence becomes clearer after 1315. By the time it became a possession of the Genoese, it was known as the Castle of Holy Cross (Castrum Sancte Crucis). It is uncertain when it actually it was handed over, though it was certainly by the year 1384. Around this time the Tower of the Holy Cross was added to the structure, over which a gilded cross was placed in 1391. A bell was also added to the fort in 1390, which perhaps was used to give warning signals. Eventually the fort was linked to the walls of Galata, during which time it also began to function as an arsenal and storehouse. It continued to have this function after the Genoese surrendered Galata following the Ottoman conquest of Constantinople in 1453.  

While the chain was used as not part of the Ottoman defenses, the building - known as Mahzen-i Sultani (“Imperial Magazine”) - retained its use as an arsenal magazine for the storage of gunpowder, as it was close to cannon foundries of Tophane established by Sultan Mehmet II. The structure was damaged by an explosion caused by lightening in the early 1500s. By the mid 16th century, a lead hipped roof was added to the restored structure known as Kurşunlu Mahzen (“Leaded Magazine”) and it was used as a granary. In 1676, the granary became the Galata Customhouse, when it functioned as the main storehouse of the Galata maritime customs. Only its walls remained after its roof burned down in 1683. In 1716, an imperial pavilion (Kurşunlu Mahzen Köşkü) was built on its southwest corner for Sultan Ahmed III. In the reign of Mahmud I (1730-1754), a sufi Naqshbandi (sufi) sheik from Damascus had a dream that led to the discovery of leaded coffins of three companions of the Prophet (Vehb bin Huseyre, Amr bin As, and Süfyan bin Uyeyne) who took part in the Umayyad sieges of Constantinople. The substructure was then converted into a mosque in 1752, during which time its round tower was used as a minaret until its collapse in 1766. After being damaged by a fire in 1819, the pavilion was restored in 1822 and was used as an office for the port authority until it was destroyed by a fire in 1870. A series of public health (quarantine) offices related to the port authority were built in the late 19th century, before the mosque was restored in 1908-1910.