Derkos (Δέρκος, modern Terkos/Durusu) was a walled city on a peninsula projecting westward into Durusu Lake (Δέρκων Λίμνη). It was around 4 km south of Philea (Karaburnu) on the Black Sea in the Thracian hinterland of Constantinople. The remains of the city’s fortification walls have partially survived. The Metropolis of Derkoi is currently a see subject to the Ecumenical Patriarch of Constantinople.

In antiquity, Delkos (Δέλκως) was a village in the civic territory of Byzantion next to a lake (lagoon) noted for its abundance of fish. When Constantinople was inaugurated as the new capital, it inherited Byzantion’s extensive rural territory that included numerous suburbs and satellite towns, such as Derkos. The founding of Constantinople led to the growth of its hinterland, as small towns were raised to the status of cities and were fortified, as was the case with Derkos by the reign of Anastasios I (491-518). Around this time, Derkos was probably made into a bishopric subordinated to the metropolis of Herakleia, though it only appears in the episcopal lists in the eighth century.

Derkos was around 40 km northwest of Constantinople, approximately 20 km east of the Anastasian Walls. It was part of a system of fortifications in Constantinople’s hinterland, which included the Anastasian Wall (Long Wall of Thrace) as well as fortified cities. Derkos and Medeia on the Black Sea, along with Herakleia and Selymbria on the Marmara coast, were likely derivative projects of the Anastasian Wall. Philea (modern Karaburun), located near the mouth of the lake (lagoon) to the north, can be regarded as a twin settlement or perhaps emporion of Derkos.

During the reign of Justinian I (527-65), Archbishop Theodosios of Alexandria and 300 Monophysite clergy were exiled at Derkos, until Theodora managed to have them transferred to the Palace of Hormisdas (Monastery of Saints Sergius and Bacchus). Derkos is documented as an archbishopric at the beginning of the 10th century. In the 11th century, Derkos was a place of refuge for the inhabitants of neighboring Philea when Eastern Thrace came under attack. Derkos came under Latin rule following the Fourth Crusade’s conquest of Constantinople in 1204.

Following an alliance with Bulgarian tsar John Asen II, John III Doukas Vatatzes (1222-54) was able to establish a permanent bridgehead in Thrace, leading to the capture of Derkos, Bizye, Medeia, and other cities from the Latins in 1247. In the spring of 1345, John VI Kantakouzenos stationed troops in Derkos and other cities in the hinterland of Constantinople in order to plunder them. Andronikos IV, the son of John V, and Savcı Bey, the son of the Ottoman sultan Murad I, joined forces in a conspiracy against their fathers. Andronikos IV went to Derkos, possibly to gather an army, but after their defeat at Pikridion, he surrendered to his father at Athyra. The Archbishopric of Derkos was elevated to metropolis by 1379. It is possible that the metropolitan seat was left unoccupied for some time in the early 15th century due to extensive Ottoman devastation of the region.

In 1421, Emperor Manuel II Palaiologos (1391-1425) assigned his sons various areas of influence, with Constantine XI taking control of the Black Sea region, including Derkos. Following a treaty with Murad II in 1424, Derkos and Mesembria were the only cities remaining under Byzantine control in this area. Derkos was finally taken by the Ottomans perhaps as early as 1452 or in February or March 1453 at the latest.

Evliya Çelebi recorded his visit to Derkos in the 1660s in his travelogue, which he mentions the earthworks in front of the city walls the Ottomans used to take the city. Around the end of the 19th century, more than 400 Christian residents lived in Derkos, many of whom were Bulgarians. At the beginning of the 20th century, a church dedicated to St. George was located in Derkos, which possibly had a Byzantine predecessor. Several artifacts, including inscriptions and steles, discovered at Derkos are now located in Istanbul Archeological Museums.

The fortifications of Derkos once ran a relatively straight line for almost 500 m, going from the northern to the southern sides of the peninsula facing the mainland. The wall is relatively well-preserved in some sections, with heights measuring around 6-7 m. However much of the wall was demolished to use as building material during the 20th century. The surviving masonry consists of medium-sized blocks with a limited use of brick. The wall is around 700 m southeast of the eastern end of the peninsula, where remains known as Manastır İskelesi (“Monastery Pier”) are located; this consists of a tower-like fortification on the lake embankment that protrudes into the lake. It apparently led to what locals called the hagiasma (“holy spring”) of St. George.