Al-Mansur (714 –6 October 775) or Abu Ja'far Abdallah ibn Muhammad al-Mansur was the second Abbasid Caliph reigning from 136 AH to 158 AH (754 AD – 775 AD) and succeeding his brother As-Saffah. He is known for founding the 'Round City' of Madinat al-Salam which was to become the core of imperial Baghdad.