Besides these family disputes, Charles had to struggle against repeated rebellions in Aquitaine and against the Bretons. Led by their chiefs Nomenoë and Erispoë, who defeated the king at the Battle of Ballon (845) and the Battle of Jengland (851), the Bretons were successful in obtaining a "de facto" independence. Charles also fought against the Vikings, who devastated the country of the north, the valleys of the Seine and Loire, and even up to the borders of Aquitaine. At the Vikings' successful siege and sack of Paris in 845 and several times thereafter Charles was forced to purchase their retreat at a heavy price. Charles led various expeditions against the invaders and, by the Edict of Pistres of 864, made the army more mobile by providing for a cavalry element, the predecessor of the French chivalry so famous during the next 600 years. By the same edict, he ordered fortified bridges to be put up at all rivers to block the Viking incursions. Two of these bridges at Paris saved the city during its siege of 885–886. Charles engaged in diplomacy with the Emirate of Cordoba, receiving camels from Emir Muhammad I in 865. From the 860s, the palace of Compiègne became an increasingly important centre for Charles and he founded a monastery there in 876. In the tenth century Compiègne was known as ‘Carlopolis’ because of its association with Charles.