The substitution of masonry for palisading upon the mound suggested a similar course for the defence of the bailey, and the twelfth century witnessed the erection of many of those gigantic walls surrounding them which excite our admiration at the present day by their massiveness and strength. They followed the scarp of the original mounds, and in many examples the water of the external fosse lapped their bases. The addition of a barbican or ravelin to defend the chief entrance to the castle, which invariably opened into the bailey, was now adopted, while the former wooden ladders or bridges giving from the motte to the bailey were superseded by causeways of stone, defended on either side by a continuation of the bailey enceinte up the slope of the mound. Stone steps instead of wood led from the inner surface of the curtain walls to the ramparts above; stone buildings were erected for the domestic offices, barracks, etc., while the wooden planks and ladders by which the moats had formerly been crossed gave place to masonry arches. These improvements in the majority of examples did not occur at the same time, hence the presence of a twelfth-century Shell Keep is no guarantee that the curtain walls are of the same age. The introduction of flanking towers, generally semicircular, into the curtain wall, and of rectangular towers, astride it, as a rule, occurred in this century. There are examples in our island, however, which prove that only partial adoption of these improvements took place in many castles, and that, for example, the baron and his family were quite content to dwell within the wooden bretasche upon the motte, at the same time strengthening the weaker bailey defences by the erection of a substantial curtain wall.