Castles were erected to serve two main purposes: to provide shelter, and to act as a fortress should the family be subjected to attack. Unlike other buildings, no standard model exists for a castle - each was as individual as the owner - but layout was usually dependent on cost, domestic needs and local materials available. Most did include a keep, or great tower, and a fortified enclosure. During medieval times the stone castle was virtually impregnable to attack but, with the advent of the cannon, many English castles were made ruinous during the English Civil War.