While Constantinople is famous for its strategic location, the triangular peninsula on which the city rested lacked sufficient fresh water sources. A large number of cisterns supplied by aqueducts were built in Late Antiquity to help supply water to the growing population of Constantinople. These cisterns include huge open cisterns or reservoirs, like the Cistern of Aetius, as well as covered or underground cisterns, like the Basilica Cistern. They are among the most impressive Byzantine structures of modern Istanbul.

Constantinople’s insufficient water sources made cisterns an essential feature of its water supply system. The city had to rely on its Thracian hinterland for water, as the city itself had no rivers, a few small springs and a long dry summer with little rainfall. The Lycus (Lykos), while often called a river, was in fact a small stream emptying into a bay that was later transformed into the Harbor of Theodosius. There were also a few natural springs that later became associated with the Virgin Mary, such as the famous holy spring (hagiasma) at the Church of Blachernai. As none of these water sources provided adequate water for city’s growing population in Late Antiquity (which perhaps was half a million in the 5th-6th century), Constantinople came to acquire an unprecedented number of cisterns. Recent research has documented more than 200 cisterns from the Byzantine era in the city, with most of them being covered cisterns built prior the 7th century. They became central to Constantinople’s water supply system and required significant maintenance. Major cisterns were usually placed on or near the top of hills to supply water to various buildings, such as public baths, palaces, monasteries, and churches. These cisterns stored the water brought in by aqueducts, as there were water shortages during the long dry summers of the region. They were also essential for the security of the city, particularly during sieges.

An incredible number of covered cisterns were built in Constantinople, including the largest and most famous underground cistern known as the Basilica Cistern. This cistern was built during the reign of Justinian I (527-565), probably during the reconstruction of the Basilica Stoa above it, which was finished in 541. It is around 138 x 65 meters and is supported by 336 columns. The second largest covered cistern is the so-called “Cistern of Philoxenos”, which measures 64 x 56 meters with 224 columns. A short distance east is one of the oldest covered cisterns - the so-called “Theodosius Cistern”, which is around 24x40 meters and has 32 columns. The first recorded covered cistern was the Cistern of Modestus, which was completed in 369. While it did not survive, it was probably the cistern used for the Ottoman Saddler’s Market known as Saraçhane. It was measured at 154 x 90 meters.

Several large open-air water reservoirs were built at high points of the city following the construction of the Theodosian Walls. The plan of the open cisterns is rectangular or square, with masonry of alternating bands of ashlar and brick. Two of these open cisterns, the Cistern of Aetius and the Cistern of Aspar, were located on the Valens line between the Constantinian and Theodosian walls. It is possible that the open cistern were used for commercial and agricultural purposes rather than for public use. The Cistern of Aetius (244 x 85 m) was probably built in 421 when Aetius served as the eparch of Constantinople. The Cistern of Aspar (152 square meters) was built in 459 by the Patrician Aspar, an influential 5th century figure of Alan descent who served as magister militum and consul. The Cistern of St. Mocius (170 x 147 meters) seems to date to the reign of Anastasius (491–518). Perhaps around the 6th or 7th century, another open cistern was constructed in the suburb of Hebdomon, and perhaps supplied the army camp and imperial palace located here. In addition, there is a massive wall in Cağaloğlu that seems to be the remains of another open cistern. 

Cisterns were supplied by a gravity-based system that brought water from the hinterland of the city. It seems to have consisted of Hadrian’s line, which entered the city just north of Kaligaria Gate (Eğri Kapı), passed near the cistern near the Pantokrator Monastery and ended at the Basilica Cistern. It also supplied the area around the Great Palace, the Baths of Zeuxippus and the Hippodrome. The Valens line probably entered the city north of Gate of Charisius (Edirnekapı), supplied the open cisterns of Aetius and Aspar, crossed the Aqueduct of Valens and terminated in the Cistern of Philoxenos. The water supply of Constantinople decreased in the 7th and 8th century as the Avars cut the Valens line during the Siege of Constantinople of 626, and was only restored around 765 by Constantine V (741-775). It is unclear how seriously damaged the Valens line was, which could have had disastrous implications for the city’s demographics. While it has generally been argued that the Valens line ceased functioning completely, it also has been suggested that the city continued to be supplied by water sources closer to the city. 

The next recorded work on the water system dates to the reign of Basil II (976-1025). Large amounts of water were required in this period, as the city possibly reached around half a million by the 11th century. Repairs were also made on the aqueducts around 1170 during the reign of Manuel I Komnenos (1043-1080). It seems that the long-distance water system was beyond repair by the 12th century, so the city possibly mostly relied on the water sources from the Belgrade Forest, as it did later during the Ottoman era. The water supply system was not repaired following the Latin conquest of Constantinople in 1204, after which cisterns became the main source of water in the city. While large cisterns required a functioning aqueduct system to supply water, it is possible that some of the smaller cisterns primarily collected rainwater through the Byzantine era. Collecting rainwater became much more important after the aqueduct lines were no longer functioning. This is likely true at many of the cisterns at monasteries, like the cisterns under Pammakaristos and Chora. It is possible that rainwater was channeled to these cisterns from the monasteries’ rooftops.

Cisterns are a possible source of a wide range of information about Byzantine Constantinople. In the Early Byzantine era, most of the large covered cisterns were built around the oldest area of the city, on the First and Second Hill near the Great Palace and the Hippodrome. The size of these cisterns seems to reflect the density and location of the population at the time. During the Middle Byzantine era, a large number of cisterns were built on the northern slopes of the Fourth and Fifth Hills of the city, which perhaps indicates that the population density was higher here during this period. There were also a significant number of cisterns around the First Hill during this period. It also seems that some cisterns also help mark the location of certain streets, as seems to be the case of the cisterns on Sofular Street.

Many cisterns were located within the enclosure of the monasteries, as seen in the cluster of cisterns around the Pantokrator Monastery. Some substructures and crypts were converted into cisterns, particularly in the Middle and Late Byzantine eras. The cistern under the naos of the Pammakaristos Monastery, for example, was possible a crypt belong to an older structure – later converted into a cistern. Many older building, including the Sphendone of the Hippodrome, the Palace of Lausus, and the Myrelaion rotunda, were eventually converted into cisterns as well. The open cisterns cease to function after 1204 and by the Ottoman era, they were used as gardens (known in Turkish as çukurbostan or “sunken garden”).  The Byzantine cisterns of the city were not used for the supply or distribution of water during the Ottoman era. However they were often used for other purposes. For example, many of the surviving covered cisterns were later used in the production of silk during the Ottoman era.