When excavated it was considered as a cistern, but later on further study revealed that is a church with a free cross plan, and it is thought that the Gospel of John, one of the four Gospels accepted as the holy book by Christians, was written here.

The Ayasuluk Basilica Cistern was a small water-reservoir, originally constructed as a basilica in the fifth or sixth century CE and converted in to a cistern during the Ottoman period. Constructed at the highest point inside Ayasuluk Fortress, it was once the apse of a Byzantine basilica-church. The cistern was supplied with rain water and it provided water for castle baths and the adjacent fountain.

Situated on Ayasuluk Hill, immediately south of the ancient city of Ephesus, the Ayasuluk Basilica Cistern formed an essential component of the water-management system of the hilltop complex dominated by the Basilica of St. John. The hill, known in antiquity as the site associated with the burial of the Apostle John, became a major religious and defensive center in the Byzantine period. Given the absence of natural springs on the summit, the survival of both the basilica and the surrounding population depended on large-scale rainwater collection and storage. The cistern reflects the continuity of Roman hydraulic engineering traditions adapted to the topographical and strategic constraints of a fortified sacred precinct.

circa 550 CE

The cistern is closely connected to the construction and use of the Basilica of St. John, erected in the reign of Emperor Justinian I in the sixth century CE. This monumental basilica, built over what was venerated as the tomb of John the Evangelist, transformed Ayasuluk Hill into a major pilgrimage destination within the Eastern Roman Empire. The development of such a large ecclesiastical complex necessitated reliable water storage for liturgical use, daily consumption, and the needs of clergy, pilgrims, and soldiers stationed on the hill.

Although the cistern itself is not explicitly mentioned in surviving literary sources, its construction is generally attributed to the same phase of large-scale building activity initiated under Justinian. Archaeological evidence indicates that the cistern continued to be used well into the Middle Byzantine period, even as Ephesus declined due to silting of its harbor, earthquakes, and shifting settlement patterns. During periods of Arab raids in the seventh and eighth centuries, Ayasuluk Hill functioned as a defensible refuge, further increasing the strategic importance of secure water reserves. The cistern likely remained in use after the Seljuk capture of the area in the late eleventh century, reflecting the long-term reuse of Byzantine hydraulic installations in Anatolia.

circa 550 CE

Exterior
Architecturally, the Ayasuluk Basilica Cistern exemplifies late Roman and Byzantine cistern design adapted to a sloping hilltop environment. It is a subterranean structure built of stone masonry, with walls rendered waterproof through thick hydraulic mortar. The brick-beamed stone wall of the apsis has a thicknessof 1.10 meter, and an addition wall was added to support this older construction and covered with red plaster.

circa 550 CE

Interior
The interior is organized into multiple bays created by rows of piers or columns supporting barrel or groin vaults, a system that distributed the weight of the overlying structures while maximizing storage capacity. The so-called basilica, covered with a wooden or vaulted roof, had a single nave. It had a narthex. When the church-basilica was turned in to the water-cistern, the nave section, narthex and other rooms located on the sides of apsis were removed and upper section was covered with a vault. The middle section was supported with two columns.

The cistern collected rainwater from the roofs and paved surfaces of the basilica complex through a network of channels and downspouts, directing it into the reservoir below. The use of spolia—reused architectural elements from earlier Roman buildings in Ephesus—is evident in the supports, a common practice in Byzantine construction that reflects both practicality and the availability of high-quality materials from the abandoned city below. Access openings and maintenance shafts allowed for cleaning and water retrieval, while the overall scale of the cistern underscores the permanent and institutional nature of the settlement it served.