A powerful fleet was built up under several admirals, or "emirs", of whom the greatest was George, formerly in the service of the Muslim prince of Mahdia. Mainly thanks to him, a series of conquests were made on the African coast (1146–1153). From 1135 Roger II started to conquer the coast of Tunisia and enlarge his dominions: Tripoli was captured in 1146 and Cape Bona in 1148. These conquests were lost in the reign of Roger's successor William, however, and never formed an integral part of the kingdom in southern Italy.