Seven out of the ten tower keeps are built on precisely the same plan as those of Henry I. The chief advance is in the masonry. All the tower keeps of Henry II., except Dover, Chilham, and Canterbury, are or have been cased with good ashlar, of stones somewhat larger in size than those used by Henry I. The same may be said of the shell walls (namely, Windsor and Arundel); it is interesting to note that Henry II. still used this elementary form of citadel, which consisted merely of a wall round the top of a motte, with wooden buildings inside.[1166] In three cases out of the ten tower keeps, Newcastle, Bowes, and Richmond, the basement storey is vaulted, which does not occur in the older keeps.[1167] Yet such important castles as Scarborough, Dover, and Canterbury are without this provision against fire. None of these keeps appear to have more than three storeys above the basement.[1168] None of the entrances to the keeps (except Tickhill) have any portcullis grooves,[1169] nor any special contrivances for defence, except at Canterbury, where the entrance (on the first floor) takes two turns at right angles before reaching the hall to which it leads.[1170] There are nearly always in the keeps of Henry II. some signs of Transition Norman in the details, such as the nook shafts at the angles of the towers of Scarborough and the Peak, certain arches at Canterbury, the Transition capitals used at Newcastle, and the filleted string round the outside of Bowes. But we have yet to speak of three keeps of Henry II.'s reign which are on a different plan to all the others, and which point to coming changes--Chilham, Orford, and Tickhill.[1171] Chilham is an octagonal tower of three storeys, with a square annexe on one side, which appears to be original. Orford is polygonal outside, round inside. Orford indeed is one of the most extraordinary keeps to be seen anywhere, and we must regard it as an experiment, and an experiment which appears never to have been repeated.[1172] Instead of the usual Norman buttresses, this polygonal keep has three buttress towers, placed between every four of the outer faces, 22 feet wide, and 12 feet in projection.[1173] Tickhill, however, the last keep he built, is decagonal. The object of the polygonal tower was to deflect the missiles thrown from siege engines, and the round tower was evidently considered an improvement on the polygonal for this purpose, as it subsequently supplanted the polygonal type. It is therefore rather remarkable that Henry II. built both these keeps in the second decade of his reign, and afterwards went on building square keeps like his predecessors. We have seen, however, that he built at least one polygonal tower in Normandy, that of Gisors. We must bear in mind that the Norman and Angevine frontier was the theatre of the continuous struggle of Henry II. with the French kings, Louis VII. and Philip Augustus, and that it is here that we must expect the greatest