Stone or Square Keep Castles were built in England by William the Conqueror. Stone Keep Castles were the extension of Motte and Bailey castles. Motte and Bailey castles were only temporary castles while stone keep castles were built to last. The most famous of the castles were the White Tower of the Tower of London and Rochester Castle in Kent.
The use of Stone allowed stone keeps to be built an entirely different way to build castles than the Motte and Bailey. Stone was stronger and allowed to builder to build up. Motte and Bailey castles were made out of a weaker wood and had and builders were limited to the size and height they could go.
Because that Stone Keep Castles were built high, this gave them an advantage to see far for enemies, allowing them to have time to set up their defences. Stone Keep Castles had other defensive mechanisms, while you could set a wooden Motte and Bailey castle alight, it was impossible to set Stone alight. William the Conqueror’s Stone Keep Castles kept their front door on the first floor, with wooden steps leading up to it. If the castle was attacked, they would set fire to the stairs preventing the enemies from reaching in.
Square Keep Castles cost a fortune to build. A king in medieval Britain had about £10,000 to spend each year; an average castle would cost about £4,000 to build.
The main part of the castle was the Keep where the Baron or Knight of the castle lived together with his soldiers and Knights.