 At the heart of Windsor Castle is the Middle Ward , a bailey formed around the motte or artificial hill in the centre of the ward . The motte is 50 feet ( 15 m ) high and is made from chalk originally excavated from the surrounding ditch . The keep , called the Round Tower , on the top of the motte is based on an original 12th @-@ century building , extended upwards in the early 19th century under architect Jeffry Wyatville by 30 ft ( 9 m ) to produce a more imposing height and silhouette . The interior of the Round Tower was further redesigned in 1991 – 3 to provide additional space for the Royal Archives , an additional room being built in the space left by Wyatville 's originally hollow extension . The Round Tower is in reality far from cylindrical , due to the shape and structure of the motte beneath it . The current height of the tower has been criticised as being disproportionate to its width ; archaeologist Tim Tatton @-@ Brown , for example , has described it as a mutilation of the earlier medieval structure . 
