Mansurah, Al (äl mänsōrˈä) [key], city (1986 pop. 317,508), N Egypt, a port in the Nile River delta. It is an agricultural market and industrial center. Manufactures include ginned cotton, cottonseed oil, and textiles. Al Mansurah was founded in 1221 to replace Dumyat (Damietta), then occupied by Crusaders. In 1250, Crusaders under Louis IX of France suffered a crushing defeat there at the hands of the Mamluks. The city is the seat of a branch of the Univ. of Cairo, the Institute of Al Mansurah (affiliated with Al Azhar Univ. in Cairo), and Al Mansurah Polytechnic Institute.