The promontory to the west of the town is crowned by the ancient citadel; now a mere shell, but imposing from its situation, and surmounted by two lofty Roman columns formerly a portion of a temple portico.[10] Towards the town the hill is precipitous, but on the further side the slope is gradual; and accordingly the whole of this face, together with the two return ends, is defended by one of the most magnificent dry moats that exists anywhere in the world. It is hewn out of solid rock, with sides that are absolutely vertical; and may measure even now about thirty feet deep and not less than thirty feet wide. Formerly it could be crossed at two or three places by narrow wooden drawbridges; and the posterns to which they gave access can still be seen in the walls. At what epoch this moat was constructed we did not feel competent to determine. The walls are partly Saracenic, partly Roman, and partly Sassanian; they are now extremely ruinous and of no very formidable height.[11] Urfa in classical days was known by the name of Edessa, and was the capital city of that king Abgarus of Osroëne, whose Epistle to our Lord is included among the Apocryphal Gospels. This tale is something more than a legend, for it dates from the beginning of the fourth century; and is related by the historians Eusebius and Moses of Khorene, who both profess to have derived their authority from contemporary documents which they had themselves inspected among the royal archives at Edessa. They tell us how the king was afflicted with leprosy, and how he sought in vain to be cured by the physicians and sorcerers of his own land. How at length he heard report of the miracles that were being wrought in Judaea by Jesus the Prophet of Galilee; and how he dispatched ambassadors to Him, entreating Him to come and heal his disease and to instruct his people, offering Him at the same time a secure asylum from the hatred of the unbelieving Jews. These ambassadors were the "certain Greeks"[12] who are mentioned in St. John's Gospel as having been introduced to our Lord by Philip on the day of His triumphant entry into Jerusalem; and they brought back to Abgarus a verbal message (or some say an actual letter dictated by our Lord to Thomas) promising that one of His Apostles should be sent to Edessa in due time. Accordingly soon after the Ascension the Apostle Thaddeus was sent by Thomas to preach the Word in Osroëne. He came and healed Abgarus of his leprosy; and the king and all his people thereupon embraced the Faith.[13] Thaddeus himself passed onwards to Armenia and Eastern Mesopotamia, where he founded the Parthian or Assyrian, now called the "Nestorian," Church.