Military engines produced little effect upon it, but a mine was opened which, says Wendover, first brought down the walls and then a part of the tower. This is what we now see. The keep seems to have been repaired in haste at once, the outer wall probably not till 1225, when Henry III. spent considerable sums upon the castle, and the capping-tower of the curtain is of that date. The attack by sap was the only one to be employed against a rectangular keep, and was rarely practicable. Where, as was often the case, the keep stood upon a rock, the running a mine below it would produce no effect. Where this was not the case, the foundations of the wall were so broad and so solidified as to stand even when much of the soil beneath them was removed. At the White Tower, for example, when it was found convenient to bring a railway from the river quay into the base of the keep for the shipment of stores, about 20 feet of solid masonry had to be cut through, and much earth removed, and this, with every aid, was found to be a very tedious and expensive operation. The defence of such a keep was its passive strength alone. The loops were nothing in its defence; the roof being on a slope and of shingle would support no military engine and no great store of stones or heavy missiles. The narrow doorway did not allow of a sally in force, and when seriously attacked the garrison had no resource but to trust to the thickness of their walls, their ample supply of water, their magazines of provisions, and thus patiently to await relief. Such are the details of the rectangular Norman keeps, of which we have in England about fifty extant or well-recorded examples, dating from the year 1078, when the White Tower was begun, to about the year 1180, to which may be attributed the keep of Helmsley. LIST (APPROXIMATIVE) OF RECTANGULAR KEEPS IN ENGLAND. _Cheshire._—Chester. _Cornwall._—Carnbrè (?). _Cumberland._—Carlisle. _Derbyshire._—Castleton. _Devon._—Okehampton (?). _Dorset._—Corfe, Sherborne. _Durham._—Norham. _Essex._—Colchester, Hedingham, Walden. _Gloucestershire._—Bristol (destroyed). _Hants._—Christchurch, Porchester, Wolvesey. _Hereford._—Goderich. _Kent._—Canterbury, Dover, Malling, Rochester, Thurnham. _Lancashire._—Clitheroe, Lancaster. _Middlesex._—The Tower. _Monmouth._—Chepstow. _Norfolk._—Bungay, Castle Rising, Mileham, Norwich. _Northumberland._—Bamburgh, Mitford, Newcastle, Prudhoe. _Nottingham._—Nottingham (destroyed). _Salop._—Bridgnorth, Clun, Ludlow, Wattlesborough. _Somerset._—Taunton. _Surrey._—Guildford. _Sussex._—Bramber. _Wales._—Ogmore, Penllyne. _Warwickshire._—Kenilworth. _Westmoreland._—Appleby, Brough, Brougham. _Yorkshire._—Bowes, Helmsley, Middleham, Normanby (?), Richmond, Scarborough. CHAPTER X. OF THE SHELL KEEP.