It is quite in accordance with this that in the narrative of Dudo, who is our only authority for the history of Normandy in the 10th century, there is no mention of a private castle anywhere. We are told that Rollo restored the walls and towers of the _cities_ of Normandy,[199] and it is clear from the context that the _castra_ of Rouen, Fécamp, and Evreux, which are mentioned, are fortified cities, not castles. Even the ducal residence at Rouen is spoken of as a _palatium_ or an _aula_, not as a castle; and it does not appear to have possessed a keep until (as we are told by a later writer) the same Duke Richard who introduced the feudal system into Normandy built one for his own residence.[200] It is possible that when the feudal oath was exacted from the more important barons, permission was given to them to build castles for themselves; thus we hear from Ordericus of the castle of Aquila, built in the days of Duke Richard; the castle of the lords of Grantmesnil at Norrei; the castle of Belesme; all of which appear to have been private castles.[201] But there seems to have been no general building of castles until the time of William the Conqueror's minority, when his rebellious subjects raised castles against him on all sides. "Plura per loca aggeres erexerunt, et tutissimas sibi munitiones construxerunt."[202] It is generally, and doubtless correctly, supposed that _aggeres_ in this passage means mottes, and taking this statement along with the great number of mottes which are still to be found in Normandy, it has been further assumed (and the present writer was disposed to share the idea) that this was the time of the first invention of mottes. But the facts which have been now adduced, tracing back the first known mottes to the time of Thibault-le-Tricheur, and the county of Blois, show that the Norman claim to the invention of this mode of fortification must be given up. If the Normans were late in adopting feudalism, they were probably equally late in adopting private castles, and the fortifications of William I.'s time were most likely copied from castles outside the Norman frontier.[203]