Of these subnations none was more important or interesting than Normandy. The Northmen had been the scourge of those who lived near the North Sea for many years before one of their leaders, Rollo (or Hrolf), agreed to accept from Charles the Simple (in 911) a district on the coast, north of Brittany, where he and his followers might peacefully settle. Rollo assumed the title of Duke of the Normans and introduced the Christian religion among his people. For a considerable time the newcomers kept up their Scandinavian traditions and language. Gradually, however, they appropriated such culture as their neighbors possessed, and by the twelfth century their capital, Rouen, was one of the most enlightened cities of Europe. Normandy became a source of infinite perplexity to the French kings when, in 1066, Duke William the Conqueror added England to his possessions; for he thereby became so powerful that his suzerain could hardly hope to control the Norman dukes any longer. [Sidenote: Brittany.] The isolated peninsula of Brittany, inhabited by a Celtic people of the same race as the early inhabitants of Britain, had been particularly subject to the attacks of the Scandinavian pirates. It seemed at one time as if the district would become an appendage of Normandy. But in 938 a certain valiant Alain of the Twisted Beard arose to deliver it from the oppression of the strangers. The Normans were driven out, and feudalism replaced the older tribal organization in what was hereafter to be called the duchy of Brittany. It was not until the opening of the sixteenth century that this became a part of the French monarchy. [Sidenote: Origin of the Flemish towns.] The pressure of the Northmen had an important result in the low countries between the Somme and the Scheldt. The inhabitants were driven to repair and seek shelter in the old Roman fortifications. They thus became accustomed to living in close community, and it was in this way that the Flemish towns--Ghent, Bruges, etc.--originated, which became in time famous centers of industry and trade. The founders of the great families of the district first gained their influence in defending the country against the Scandinavian pirates. The counts of Flanders aspired to rule the region, but the lesser counts within their territory were pretty independent of them; so private wars were frequent and bloody. [Sidenote: Burgundy.]