William probably started to develop Caen as a major town and fort in answer to the rebellions by the barons of Lower Normandy (particularly at Val-ès-Dunes in 1046-1047).
It is notable that he chose Caen for the sites of the abbeys which he and his wife Matilda founded, supposedly in penance for their marriage, which Rome initially opposed on unknown grounds (thought to be consanguinity).
The abbeys rise up from hills either side on the castle; a clear message that Lower Normandy was ruled by the Duke of Normandy under God's laws.
The Ducal Castle as it stands today
Its oldest remaining building is the exchequer, built by William's 4th son, Henri Beauclerc
The vestiges of William's buildings
William's Abbaye aux Moines - St Etienne (St Stephen's abbey for monks)
William's tomb inside the abbey
Matilda's Abbaye aux Dames - Ste Trinité (Holy Trinity abbey for nuns)
Matilda's tomb in the abbey