 Windsor Castle was originally built by William the Conqueror in the decade after the Norman conquest of 1066 . William established a defensive ring of motte and bailey castles around London ; each was a day 's march – about 20 miles ( 32 km ) – from the city and from the next castle , allowing for easy reinforcements in a crisis . Windsor Castle , one of this ring of fortifications , was strategically important because of its proximity to both the River Thames , a key medieval route into London , and Windsor Forest , a royal hunting preserve previously used by the Saxon kings . The nearby settlement of <unk> , or <unk> , was an old Saxon residence . The initial wooden castle consisted of a keep on the top of a man @-@ made motte , or mound , protected by a small bailey wall , occupying a chalk <unk> , or bluff , rising 100 ft ( 30 m ) above the river . A second wooden bailey was constructed to the east of the keep , forming the later Upper Ward . By the end of the century , another bailey had been constructed to the west , creating the basic shape of the modern castle . In design , Windsor most closely resembled Arundel Castle , another powerful early Norman fortification , but the double bailey design was also found at Rockingham and Alnwick Castle . 
