The above works, curtain, gateways, observatory tower, and keep contain the only Norman masonry now extant. The curtain, for its great length, is singularly deficient in flanking towers. At the south-west angle the wall has been laid open by a wide breach, and built up, and there may have been a tower, but it is more probable that here was the junction with the city wall. Generally, the lower two-thirds of the curtain look much older than the upper part, and the line of junction is very uneven, as though the wall had long been left in a ruinous condition. This may have been so; but, as there is no diminution in the excessive thickness, it seems more probable that the new work is confined to the facing. No doubt when the castle was taken by the county much was done to the walls; but they could scarcely, at that time, have been much lower than they are now, for to rebuild them at their present thickness would have been a great and quite unnecessary expense. The herring-bone work is of a superficial character, confined to the facing; not, as in the Roman work, carried through the substance of the wall. Mr. Wilson, in his paper on this castle, states that, when the foundations of the curtain were laid open during some repairs, they were found to be worked in with a sort of framework of timber, tieing the whole together. Such a precaution was often taken by the Norman builders, even to the extent of enclosing ties in the superstructure, especially where the work was laid upon made ground. The cavities left by the decay of such ties are seen at Rochester and elsewhere. There is a flanking tower capping the north-east angle of the place, an insertion, though whether replacing a Norman structure is not known. It is called “Cobbe Hall,” and is in plan very slightly horseshoe, with prolonged sides and a square rear. It is in breadth 25 feet, and in length 40 feet. It has a basement and first floor, both covered in with acutely-pointed vaulting, with deeply-splayed loops towards the field. The basement is reached by a trap-door and ladder, and the upper floor and battlements by a stone stair. It seems, from the rings let into the wall, to have been a prison. It has been called a chapel, probably because its round or apsidal end looks towards the north-east. There is a deep well in the north side of the great enclosure, still in use, and the bottom of which has recently been enlarged into a cistern. The castle stands upon the oolite rock, and is mainly built of that material, laid as roughly-coursed rubble, but the keep, the observatory tower, and the Decorated work are mostly of ashlar.