After dynasty related quarrels however, there was a Christian reconquest in the form of the Normans from the Duchy of Normandy, descendants of the Vikings; the family who returned the city to Christianity were called the Hautevilles . Palermo was conquered in 831 by Arabs from North Africa and became the capital of the Arabian Emirate of Sicily until 1072, when it returned under Christian rule due largely to the efforts of Robert Guiscard and his army. For more than two hundred years Palermo was the capital of a flourishing Islamic civilisation in Sicily. By 1050, Palermo had a population of 350,000, making it one of the largest cities in Europe, 3rd only to Constantinople with 600,000, and Islamic Spain's capital Córdoba, which had a population of 450,000.