The wall was usually 8 feet to 10 feet thick, and, as a security against settlement, was generally placed 2 feet to 3 feet within the edge of the mound. Sometimes, as at Lincoln and Tickhill, the walls were strengthened by flat Norman pilasters, but more commonly they were of plain rubble with plinths and angle quoins of ashlar. Cardiff, though without a plinth, has angle quoins, and probably dates from early in the twelfth century. The walls of these keeps were 20 feet to 25 feet high to the rampart walk, which was reached by open interior staircases either of wood or stone. Traces of the latter were visible a few years ago at Cardiff. Such a shell is well suited to be placed upon made ground, which would not have supported a solid mass. At Cardiff, where a heavy tower was long afterwards added, it slipped and fell. The interior of the larger keeps was an open court, around which were placed the buildings—sometimes mere sheds—against the ring wall. Where the keep was large and the castle important, these buildings were permanent, though even in the greater keeps, such as York, timber was largely used. Windsor, before the alterations, contained an open court, as still does the keep of Ledes Castle, which is peculiar in standing upon a small island, completely occupied by it. The present round tower at Windsor, though modern, is built upon the lines of the Edwardian tower, which again was built upon those of a late Norman keep, the foundations of which were laid open by Wyattville, and found to rest upon an artificial mound. Durham keep is said to have been originally open, but to have been closed to accommodate the bishops, who were forced, by the disturbed state of the country, to reside within it. The keep at York, known as Clifford’s tower, is rather Early English than Norman, and probably succeeded a timber structure. It much resembles some of the French keeps built by Philip Augustus. It has a well in the court, turnpike stairs to the ramparts, and a chapel over the entrance, much altered in the Decorated period, and which, as is not uncommon, serves as a portcullis chamber. At Hawarden, a small and close, but very curious keep, the oratory is over the entrance.