The characteristic types of castles in the twelfth century were the rectangular keep and the shell keep; in the thirteenth the concentric castle. Of the two last we have splendid examples in Cardiff and Caerphilly. Of rectangular keeps there are very few in Wales--Chepstow is the only important one--though there are several on the borders, notably Ludlow. The square keep seems to us most characteristic of Norman military architecture; the Tower of London, Rochester, Newcastle, Castle Rising, are well-known examples, and there are many more in a good state of preservation; there are many more solid square keeps than shell keeps well preserved, but this is simply due to the greater solidity of the former; the shell keeps were far more numerous in the twelfth century; and the reasons for this are obvious--the rectangular keep was much more expensive to build, and it was too heavy to erect on the artificial mounds on which the Norman architects generally founded their castles.
The keep of Cardiff Castle is one of the most perfect shell keeps in existence. It is built on a round artificial mound, surrounded by a wide and deep moat--the mound and moat being, of course, complements of each other. Such mounds and moats are common in all parts of England, and in Normandy. They are not Roman, nor British, nor are they, as Mr. G. T. Clark maintained, characteristic of Anglo-Saxon work. They are essentially Norman, and a good representation of the making of such a mound may be seen in the Bayeux Tapestry, under the heading--'He orders them to dig a castle.' When was the Cardiff mound made? Perhaps the short entry in the Brut gives the answer: "1080, the building of Cardiff began." It would then be surrounded by wooden palisades, and surmounted by a timber structure, as a newly made mound would not stand the masonry. The shell keep was probably built by Robert of Gloucester, and it was probably in the gate-house of this keep, that Robert of Normandy was imprisoned. A shell keep was a ring wall eight or ten feet thick, about thirty feet high, not covered in, and enclosing an open courtyard, round which were placed the buildings--light structures, often wooden sheds, abutting on the ring wall--such as one may see now in the courtyard of Castell Coch. The shell keep was the centre of Robert's castle, but not the whole. From this time dated the great outer walls on the south and west--walls forty feet high and ten feet thick and solid throughout. The north and east and part of the south sides of the castle precincts are enclosed by banks of earth, beneath which, the walls of a Roman camp have recently been discovered. These banks were capped by a slight embattled wall. Outside along the north, south and east fronts was a moat, formerly fed by the Taff through the Mill leat stream which ran along the west front. The present lodgings, or habitable part of the castle built on either side of the great west wall, date mostly from the fifteenth century. The earlier lodgings were, perhaps, on the same site--though only inside the wall; a great lord did not as a rule live in the keep, except in times of danger.