It is significant of the progress made by the ducal authority in Normandy that by the time of William the Conqueror definite restrictions had been placed upon the creation of these strongholds of local power and resistance. Except with the duke’s license no one could build a castle, or erect a fortress on a rock or an island, or even dig a fosse in the open country so deep that the earth could not be thrown out from the bottom without artificial aid, while palisades were required to be built in a simple line and without alures or special works of defence. When the duke desired, he might also place garrisons in his barons’ castles and demand hostages for their loyalty. These principles, which were applied also in England, were of course often difficult to enforce, and they were supplemented in the twelfth century by the development of a great system of ducal castles, secured partly by enlarging and strengthening the older fortresses of Rouen, Caen, Falaise, and Argentan, partly, as we have already seen, by new strongholds on the frontiers. Powicke has shown us how these castles became the chief administrative centres in the reigns of Henry II and Richard, and how the royal letters and accounts reveal their many-sided activity in the busy days of peace as well as in the more strenuous times of war.[50] Under _châtelains_ who were royal officers rather than feudal vassals, with garrisons of mercenaries and retinues of knights and serjeants, clerks and chaplains and personal servants, they foreshadow the ultimate replacement of baronial donjons by a royal bureaucracy. It is doubtless because of the dominant position of the duke that Normandy is less rich than some other parts of France in picturesque types of feudal lords or vivid episodes of feudal conflicts. When they go beyond the affairs of the church, the Norman chroniclers are prone to concentrate their attention upon the deeds of the ducal house, and their accounts of the great vassals tend to be dry and genealogical. The chief exception is Ordericus Vitalis, whose theme and geographical position lead him to treat at length the long anarchy under Robert Curthose and the incessant conflicts of the great lords his neighbors on the southern border, the houses of Bellême, Grentemaisnil, Conches, and Breteuil. In the main it is a dreary tale of surprises and sieges, of treachery and captivity and sudden death, relieved from time to time by brighter episodes--the lady Isabel of Conches sitting in the great hall as the young men of the castle tell their dreams; the daily battle for bread around the oven at the siege of Courcy; the table spread and the pots seething on the coals for the lord and lady of Saint-Céneri who never came back; the man of Saint-Évroul who, by the saint’s aid, walks unharmed out of custody at Domfront; the marvellous vision of the army of knights and ladies in torment which appeared to the priest of Bonneval.