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 318 Medieval Military Architecture, shorter, and almost parallel to it; the northern is prolonged, and curves around its point, until it is separated from the southern only by an inconsiderable gorge. The swamp thus assumes something of the figure of a horse-shoe. South of the peninsula, the Nant-y-Gledyr, a large rivulet, flows from the south-west, across the swamp, through the gorge, to join the Rhymny. North-east of the peninsula a smaller spring, partly fed by the Nant-y-Gledyr, flows across a part of the northern swamp ; and, north of this again, another spring contributes to the same. Natu- rally, these waters seem to have found their way, by a depression or gorge, to the north-eastward, into the lower part of the Nant- y-Gledyr, outside of and below the upper gorge already mentioned. The tongue of land thus guarded was well suited for the purposes of defence, supposing the peninsula to have been converted, by a cross-trench, into an island. Water was abundant, pasturage at hand, and the morass would form a secure front. There is, how- ever, no evidence that the spot was occupied by the Welsh, though it has been supposed that the stronghold of Senghenydd was here situated. When the castle was constructed the surface of the ground under- went considerable alteration. The first business of the Norman engineer was to provide for the protection of his fortress by a large expanse of water. To effect this, the bed of the Nant-y-Gledyr was dammed up at one gorge, and the northern waters at the other, and the two divisions of the swamp thus formed into lakes, the southern of about .13 acres, the northern of about one or two. Advantage was taken of a narrow and curved ridge, which pro- ceeded from the root of the peninsula, to divide the northern water into two parts, of which the one formed the middle, and the other the inner, moat. The inner moat communicated with the southern lake by two cross cuts j one, the old natural termination of the peninsula east- wards, the other, an artificial cut across it on the west ; and thus the circuit of the inner moat was completed. The island which was thus formed, and encircled by this moat, was scarped into curtains and bastions, and faced with stone and the single cross-cut westward, not being deemed a sufficient defence, the peninsula was divided by a second cross-cut further westward, and the second island, thus formed, was converted into a sort of large horn-work or demi-lune, covering the western approach. This also was scarped and revetted Thus, then, the principal features of the ground plan are — the end of the peninsula converted into an island, and defended on the north by the inner north moat, on the south by the lake, on the east by the imier east moat, and on the west by the inner cross-cut — the whole making up the inner moat. Proceeding outwards, we have, as the boundaries of this moat — on the west, the horn-work, prolonged on the north into the curved