Harran, known to the Romans as Carrhae, is an ancient town in northern Mesopotamia along a small tributary of the Euphrates. It is located today in a fertile plain some kilometers south of the city of Urfa, very close to the Turkey-Syrian border.
Predating the rise of Islam by thousands of years, the town of Harran was known as the birthplace of Abraham and also as the center of a Hellenistic pagan cult of the sun. The city flourished during the early Islamic period. During the reign of the Umayyad caliphs, Harran became the seat of a school for medicine and also the residence of the last Umayyad caliph Marwan II (r. 691-750). The town's importance as an intellectual center continued during the Abbasid period, when local residents participated in the state-sponsored translation of Greek texts into Arabic. The pagan population had a familiarity with the traditions of Greek philosophy, astronomy and mathematics, and were thus poised for this work.
After a period of occupation by regional dynasties, Harran fell under the rule of the Zangids and Ayyubids who beautified the city and restored some of its buildings. This period of flourishing ended after the Mongol invasions, when Harran surrendered to Hulegu Khan. In 1271/ 670 AH, the people of Harran, the Ilkhan overlords of Mesopotamia deported the people of Harran to Mardin and Mosul, demolishing the city's public structures.
Although a small agricultural village exists among the ruins today, the city never returned to its former status. The town is known today for its vernacular architecture, namely its houses with beehive-shaped domed brick roofs.
The ruins from the Islamic period include 1) remnants of city walls, gates and towers, 2) a great mosque known as Jami' al-Firdaws, 3) the mausoleum of Shaykh Hayat, thought to commemorate the birthplace of Abraham's father, and 4) a citadel.
Sources:
G. Fehérvári. "Ḥarrān." In Encyclopaedia of Islam, Second Edition. Edited by: P. Bearman, Th. Bianquis, C.E. Bosworth, E. van Donzel, W.P. Heinrichs. Accessed May 9, 2017. http://dx.doi.org/10.1163/1573-3912_islam_COM_0270.