The approach to these keeps seems, in its simplest form, to have been by a wooden bridge over the ditch on the side within the castle, and thence by steps up the mound, as at Lincoln, where, however, they are modern, and the ditch is partially filled up. At Tickhill, and at Hawarden, there is a flight of steps just within, and built against the curtain. At Cardiff recent excavations have disclosed the piles of a timber bridge, which crossed the ditch, and may be older than the remains of the Norman keep. Cardiff, however, thanks to the same excavations, affords an excellent example of the more elaborate approach to a Norman shell keep. Here a thick curtain-wall traversed the court of the castle, and crossing the ditch of the mound was continued up its slope; just within the curtain was a drawbridge across the ditch, beyond which a direct flight of steps led up to the keep. Here, as at Wallingford, the well was on the slope of the mound, just within its ditch. Usually the actual entrance to the keep was by a mere door in the wall, as at Arundel, Lincoln, Tickhill, and Tamworth. At York is a regular gate-house, a part of the shell, which was reached by a very steep bridge, crossing a very formidable wet ditch. It has been mentioned that these shell keeps, and the mounds on which they stand, are usually upon, and form a part of the _enceinte_ or line of the outer defence of the castle, though isolated from it by their proper ditch, of which the main ditch of the place is commonly a part. This is well shown in the old plans of York Castle, and may be actually seen at Arundel, and many other works both large and small, and in other mounted mounds, such as Hên-Domen, near Montgomery, and Brinklow, which have never been defended by masonry. It was, of course, necessary to carry the palisade, and afterwards, if constructed, the curtain, across the ditch and up the slope of the mound to the wall of the keep, and even where the mound was central, as at Pickering and Cardiff, it was upon the division line of the two wards, and, therefore, on the line of a curtain. At Berkhampstead the keep seems to have been, not upon, but outside the line of defence, and to have formed a sort of spurwork connected with the main curtain by a single but very thick wall. Arundel, Durham, Lincoln, Tamworth, Tickhill, Tonbridge, Windsor, and Wallingford, are, or were, good examples of shell keeps on the line of the _enceinte_ wall.