Papa romano, successor of Sergio II, who was elected in 847 and died in 855. Shortly before dying Sergio II, the Saracens had devastated the Roman countryside and looted and desecrated the basilicas of San Pablo and San Pedro. Leon IV made to surround the city on the other side of the Tiber from a wall with towers to defend the Church of San Pedro and the part of the population in the districts of foreigners, the scholae of the Frisians, longobardos, Saxons and Franks, in the shelter of an invasion; called this city Leonina enclosure; He/She did repair the walls and gates of Rome and built 15 towers. He/She tried to ally himself with the inhabitants of Naples, Gaeta, and Amalfi and fought against the Saracens. He/She founded Lviv, the current Civitá Vecchia. He/She was jealous of ecclesiastical discipline, celebrating at least four councils in Rome. He/She excommunicated the cardinal Anastasius the librarian and fought against the abuses of the powerful, condemning to death two missi Emperor Luis II, although they were later forgiven and be downgraded them worthwhile thanks to the intervention of the emperor. Between his pontificate and Benedict III, his recognized successor, is placed the election of the Pope Joan.