One or two keeps have buttresses of bold projection, greatly in contrast to the usual flat pilaster. This is so at Colchester and at Arques, where the exterior stair passes through one of them. At Arques also these buttresses are turned to account in the upper story, arches being thrown across from buttress to buttress, upon which are built chambers, and on one face a chapel, through the floor of which missiles could be dropped upon the assailants below. Arques, unfortunately, is built of chalk and flint with little or no original ashlar, and it is, in consequence, difficult to decide between what is original and what has been added. Norman keeps differ in workmanship as in material. The White Tower, built in great haste, is of rubble, rudely coursed, with very open joints; but the plinth, quoins, and pilasters seem to have been of Kentish rag, dressed as ashlar, and also open jointed. Malling is an excellent example of very early Norman rubble, with open joints, and this may also be said of a part of the adjacent abbey church, and perhaps of the tower of the parish church. Guildford contains a good deal of herring-bone work; Chepstow and Penllyne a little. Colchester is partly built of old Roman materials, chiefly brick, and contains some herring-bone work. In the chalk districts flint was largely used, as at Bramber, Dover, Hastings, Canterbury, Thurnham, Berkhamsted, Bungay, and Walden. In the south, or near the sea, the ashlar is often in small blocks from Caen. Corfe is of excellent local ashlar, as is most of Kenilworth. Porchester is of chalk and flint rubble, faced with ashlar outside and partially inside. Hedingham is all ashlar, and altogether the finest keep in England. Bowes is a fine example of ashlar, in a local stone. Whatever Norman masonry may be in church towers, in keeps it is always sound, though often rough, and is very durable. Now and then chain courses of timber are inserted in the heart of the walls, to hold the work together till the mortar shall have set, and it has happened that the wall has been breached and the exposed timbers have been found to have rotted away, leaving cavities, as at Rochester, concerning the use of which much nonsense has been written.