The rectangular keep is, of all military structures, the simplest in form, the grandest in outline and dimensions, the sternest in passive strength, the most durable in design and workmanship, and in most cases by some years the earliest in date. Of the five great fortresses which covered the road from Dover to London, Dover itself, Canterbury, Rochester, and London have square keeps; the fifth, Tonbridge, having an earlier mound, has a shell keep. Farnham and Berkhampstead, Windsor, Wallingford, and Oxford,—fortresses guarding London from the west,—having mounds, have not square keeps. Hedingham and Colchester to the east, having square keeps, are without the mound, though Hedingham, like Corfe and Bramber, stands on a natural hill. Northwards, Warwick and Kenilworth confirm the rule, one having had a mound and no square keep; the other a square keep which has absorbed the mound, if mound there was. At Belvoir, the mound, a natural hill, is capped by a shell, as were the artificial mounds of Bedford, Cambridge, Clare, Eye, and Ongar, while rectangular keeps are, or were, found at Chepstow, Ludlow, Bristol and Wattlesborough, Lancaster and Newcastle, and on the moundless sites of Bamburgh, Carlisle, Corfe, Norham, Norwich, Nottingham, Porchester, Scarborough, Knaresborough, Helmsley, and Richmond, and in the less distinguished lordships, some of Norman foundation, of Appleby, Brough, Brougham, Bowes, Castle Rising, Clitheroe, Castleton, Goderich, and Prudhoe. Where there is a rectangular keep it rises high over every part of the fortress, and gives, as at Bamburgh, unity and grandeur to the architectural composition. It is usually, as at Rochester, Dover, Richmond, and Scarborough, placed upon the highest ground within the enclosure, and very rarely indeed in or near the centre, although, as in London, it may have been rendered central by the removal of its original _enceinte_ and the substitution of new and extended lines. At Malling, perhaps the earliest in England of these keeps, and at Helmsley, probably the latest,—for that of Knaresborough, though square, is of Decorated date,—the keep stands on one side of, and forms a part of the _enceinte_. At Bamburgh, Bramber, Canterbury, Carlisle, Clitheroe, and Rochester, the keep stands clear of, but very near to, the outer wall, of which, at Porchester, it forms one angle. At Kenilworth and Bridgenorth the keep forms an angle of the inner wall; at Norham one face ranges with the outer and one with the inner wall; and at Ludlow the keep is placed on the inner wall, close to the main gateway. At Ogmore it occupies an angle, having one face on the outer and one on the cross or inner wall. Dover stands detached in the middle of what appears to be its original enclosure, and so, probably, did Hedingham and Castle Rising. At Mitford, near Morpeth, is a singular example of a rectangular keep, of which three angles are right-angled, and the fourth face is broken into two planes meeting at a low salient.