When Duke William of Normandy invaded England in 1066, the existing type of fortification called a burh was a moated hillock, either wholly or partly artificial, surmounted by a timber stockade enclosing a wooden house or tower. He repaired and enlarged many of the existing strongholds, and also built many new wooden castles. But in order to overawe the conquered English he erected in the larger towns square stone keeps or castles, like the White Tower in the Tower of London. Gundulph, Bishop of Rochester, the great architect in the reign of William I., built the latter in 1081 A.D. It is one of the best examples that we have of Early Norman work, and is a huge quadrangular structure more than 100 ft. square, built of rudely coursed rubble, with a vast amount of mortar.

Many of the existing Norman keeps were founded in the reigns of William I. and William II., but were rebuilt at a later period. These keeps were usually square or rectangular towers of stone (not having much height in proportion to the breadth), with small, slightly projecting square turrets at each angle, and one or more flat buttresses up the centre of each face, with a dividing wall passing up through the centre of the building from the ground. (Pl. 22, Fig. 3, B C.)

Comparatively speaking, the windows were generally small, and the walls exceedingly thick. Those of the White Tower are 15 ft., and of Carlisle Castle 16 ft. thick. The connecting passages and staircases were constructed in the thickness of the masonry.

Norwich Castle, for its size, is a perfect type of the square Norman keep, and Castle Hedingham is another. Many magnificent stone keeps were built or rebuilt in the reign of Henry I., such as Rochester, Newark, Corfe, and Chepstow. Henry II. was also a great builder of keeps, and those of Dover, Canterbury, Scarborough, and Newcastle are shown by the Pipe Rolls to have been his work.

The solidarity of the keep made it impregnable against the siege operations of the day. Such a building could not be battered down, and at best it could only be injured by undermining. This was done by removing the earth from an angle of the building and gradually introducing wooden props. A fire was then kindled about them, and as the props burnt through, the wall fell.

The square keep was followed by the polygonal and the round ones. Coningsburgh is circular, Berkeley is circular flanked by four towers, and Oxford is polygonal. The base of the keep was generally “battered”—i.e., sloped outwards (see Fig. 1)—to give a firmer foundation and also that it might better withstand the operations of the sapper. Very few of the existing keeps have openings in the lower storeys, which were used either as dungeons or store rooms, and were only accessible by a trap-door from above. Generally there were two floors, occasionally three floors above the basement. One of these floors is assumed to have been the hall, and in the larger keeps the floor above it may have been reserved for the use of the ladies of the household. In the latter half of the twelfth century small mural chambers, probably for use as bedrooms, became more frequent.

In many keeps there were chapels, but every one contained a well, so that when besieged the occupants would not have to depend on outside sources for the supply of water. Dover Castle still has a well which is capable of supplying fresh water, although the keep is on the summit of a high cliff.

There were no kitchens in Norman keeps, as the cooking was either done on the roof or in a special building in the yard. Fire-places were not invariable. In the White Tower, where for a long time it was supposed that there were no fireplaces, holes in the wall have been found which probably answered the purpose of chimneys. There was only one entrance, perhaps some 20 feet above the ground, sometimes approached by a removable wooden staircase, sometimes by a fore-building with elaborate precautions for defence. The only ornamentation to be found is at the entrance doorway, on the staircase, or in the chapel. The keep soon had outer defences added to it—ditches, palisades, and outer walls of masonry.

Into the outer defences the cattle and stores from the surrounding country would be brought, and the dwellings of the soldiers of the garrison, together with the domestic offices and stables, were erected within these.

It is more usual to find the keep at one end than in the centre of the system of walls, and the whole was surrounded by a moat.