Which caliph built Baghdad as the new Abbasid capital?

The correct answer is Al-Mansur. He was the second Abbasid caliph and is credited with founding the city of Baghdad in 762 CE. He chose the site strategically along the Tigris River, and under his rule, Baghdad was planned and built as a round city known as Madinat al-Salam (“City of Peace”). It soon grew into one of the most important cultural and intellectual centers of the Islamic world.