Edessa

thumb|400px|Upper Mesopotamia and surrounding regions during the [[Early Christian period, with Edessa in the upper left quadrant]] Edessa (; ) was an ancient city (polis) in Upper Mesopotamia, in what is now Urfa or Şanlıurfa, Turkey. It was founded during the Hellenistic period by Macedonian general and self proclaimed king Seleucus I Nicator (), founder of the Seleucid Empire. He named it after an ancient Macedonian capital. The Greek name (Édessa) means "tower in the water". It later became capital of the Kingdom of Osroene, and continued as capital of the Roman province of Osroene. In Late Antiquity, it became a prominent center of Christian learning and seat of the Catechetical School of Edessa. During the Crusades, it was the capital of the County of Edessa.

The city was situated on the banks of the Daysan River (; ; ), a tributary of the Khabur, and was defended by Şanlıurfa Castle, the high central citadel.