Castles vary in size but the average size was around 450,000sq. feet. Around the 11th Century the walls of castles were typically built of wood and timbers. Then they started making them out of brick and stone.
Shell Keep castles were built in the year 1172! +++ Not sure where the adjective "shell" comes from, but castles with a keep - the central building in the complex, and the main accommodation - were built mainly in the few centuries of the Mediaeval period , not just in one arbitrary year.