Surveys on urbanization and rural settlements in the Late Antique and Byzantine periods have been conducted in Rough Cilicia since 2006. These surveys have shown that farm settlements comprise production areas, buildings and churches. The latter are usually basilican in layout. In addition to churches at the Late Antique and Byzantine farm settlements at Felicek Mevkii and Hisar Mevkii of Küstüllü village in Erdemli, Mersin, tetraconch buildings have also been identified. This is of importance for Christian architecture in Anatolia, particularly in Cilicia, because structures with central layouts are scarce when compared to churches displaying various other layouts.The structures at Felicek and Hisar are located within the Roman settlement, and their layout features a four-leaf clover inscribed in a square. On the interior and exterior surfaces of the walls are cut stones of various sizes, while the corners joining the conches are built with large cut stones. In the corners of the Felicek example are rooms almost square in size. Doorways between these corner rooms and the conches constitute narrow, short corridors. The west conch has one doorway, while the north and south conches have two each. The east conch does not have any at all.The structure at Hisar was adjoined with another structure almost square in size. This annex with barrel vaulting has a doorway on the*northwest, a round arched niche in its southwest wall and an embrasure window in the northeast. The walls, particularly in the niche, have traces of plaster. The main building is accessed via the doorways in the southeast and northeast, while the southwest and northwest sides do not have any openings. The northeast conch connects with the annex building via a corridor.Apart from the Felicek and Hisar examples circumscribed with a square on the outside, other similar examples in Cilicia of a tetraconch inscribed in a square layout have been found in the Milvan Fortress (Meluos Kastron), at Anazarbos and in the naos of the cemetery church at Korykos. Outside the region, the tetraconch building at Centcelles, Tarragona (first half of the 4th century); the cave church at the pilgrimage site of St Menas; those at Avan and Aramous; and the one in the bishopric castle in Zarizyngrad (Caricin Grad) have the same plan features. The courses of stones over the northeast and northwest conches of the Hisar building constitute the archaeological evidence for the superstructure of conches and indicate a semi-dome. The Felicek building's conches would also have been covered with semi-domes. Thick corner walls of the central areas at Felicek and Hisar suggest that they had a dome soaring over the core just like the Armenian-Georgian tetraconches as well as the one at Milvan Fortress in the Cilician plains. Armenian-Georgian tetraconches with corner rooms, like those at Felicek and in other regions, usually have a vaulted superstructure covering the corner rooms. The corner rooms of the building at Felicek would have been covered with vault.In the example at Felicek the corner rooms on the west side seem to have provided access to the conches only. The two on the east side seem to have assumed some liturgical function as well due to their proximity to, and connection with, the bema. Therefore the structure at Felicek may have been used as a church despite its proximity to a large basili-can church. If a baptismal font is uncovered in situ in future excavations, the building may have been used as a baptistery.The round, arched niche in the southwest wall of the annex building northeast of the tetraconch structure at Hisar may have been an arcosolium. It is very curious that the eastern conches have doorways, and the northeast conch communicates with the annex through a corridor. In case this tetraconch building was the martyrium of a holy person, then the arcosolium may have belonged to the person who commissioned the building. There are two possibilities for the faithful visiting such a site. The first is that the visitor entered the tetraconch via the southeast conch, visited the tomb of the holy person and made supplications before continuing to the corridor via the northeast conch. At this point the visitor could then have left the site or paid a visit to the tomb of the patron in the annex. The second option would be to reverse the order of the visit. It is also likely that the building at Hisar could have served as a church as well.Other than their layouts, there is no evidence like capitals to help in dating the tetraconch structures at Felicek and Hisar. The materials used in both structures and the accentuation of the interior moulding at Hisar are common features of religious structures in Cilicia and Isauria that date to the second half of the 5th century to the 6th century.The layout and architectural sculpture of the churches at Felicek and Hisar, communities densely settled as of the 2nd and 3rd centuries AD, suggest a date in the second half of the 5th century and early 6th century. These tetraconch structures with their square circumscription are two rare examples in Rough Cilicia that continue the centrally planned layouts of the Mediterranean region during this period.