blow was the death of Queen Matilda on 2 November 1083. William was always described as close to his wife, and her death would have added to his problems. Maine continued to be difficult, with a rebellion by Hubert de Beaumont-au-Maine, probably in 1084. Hubert was besieged in his castle at Sainte-Suzanne by William's forces for at least two years, but he eventually made his peace with the king and was restored to favour. William's movements during 1084 and 1085 are unclear – he was in Normandy at Easter 1084 but may have been in England before then to collect the danegeld assessed that year for the defence of England against an invasion by King Cnut IV of Denmark. Although English and Norman forces remained on alert throughout 1085 and into 1086, the invasion threat was ended by Cnut's death in July 1086. William as king Changes in England As part of his efforts to secure England, William ordered many castles, keeps, and mottes built – among them the central keep of the Tower of London, the White Tower. These fortifications allowed Normans to retreat into safety when threatened with rebellion and allowed garrisons to be protected while they occupied the countryside. The early castles were simple earth and timber constructions, later replaced with stone structures. At first, most of the newly settled Normans kept household knights and did not settle their retainers with fiefs of their own, but gradually these household knights came to be granted lands of their own, a process known as subinfeudation. William also required his newly created magnates to contribute fixed quotas of knights towards not only military campaigns but also castle garrisons. This method of organising the military forces was a departure from the pre-Conquest English practice of basing military service on territorial units such as the hide. By William's death, after weathering a series of rebellions, most of the native Anglo-Saxon aristocracy had been replaced by Norman and other continental magnates. Not all of the Normans who accompanied William in the initial conquest acquired large amounts of land in England. Some appear to have been reluctant to take up lands in a kingdom that did not always appear pacified. Although some of the newly rich Normans in England came from William's close family or from the upper Norman nobility, others were from relatively humble backgrounds. William granted some lands to his continental followers