When were stone castles built?

Stone or square keep castles were first built in Medieval England by William the Conqueror. Stone keep castles were the natural extension of motte and bailey castles. Motte and bailey castles were only temporary features (though many mottes exist to this day) while stone keep castles were built to last. (this was copyed and pasted off the website below.)

Go to www.historylearningsite.co.uk/stone_keep_castles.htm for more information.