The Şanlıurfa acropolis is situated on an extension of the Topdağı. At one time the winter palace of Abgar the Great stood here. The two Corinthian pillars may date from early-Christian times, they are called Throne of Nimrud. The inscription however only mentions Queen Šalmath. T.A. Singer claims the earliest version of the fortress dates from 812, when much of the region was under Abbasid control. It was torn down in 1235 by the Ayyubids, then in the 14th century the Mamluks rebuilt it, the Akkoyunlu under Uzun Hasan restored it and then the Ottomans did some work on it. The walls are still from 812.
As for Abgar: The Wikipedia has some interesting information about him: Lucius Aelius Megas Abgar IX was a Syriac ruler of Osroene from AD 177 to 212. Andrew Louth in his "Who's Who in Eusebius" at the end of G. A. Williamson's translation of Eusebius of Caesarea's Ecclesiastical History gives the dates of Abgar's reign as from 179-214. During the reign of Abgar the Great, Christians were favored in the realm of Osroene. It is thought by some that this led to the story of the letters between Abgar V and Jesus of Nazareth. Adolf von Harnack and many later scholars believe a corruption of a story of Abgar IX corresponding with Pope Eleuterus was the origin of the legend of King Lucius of Britain, who supposedly introduced Christianity to his realm.