In the bank of limestone rock below the bridge, and on the side of the river, is a cavern or subterraneous passage, of unknown extent, and which I have not had opportunity to explore. About Crogen Isaf, and near a bridge of very capacious span across the Ceiriog, called Pont Madoc, a powder mill was about to be erected; but when the work was nearly completed, the projector became unable to proceed, and it was discontinued. Thinking it best not to break the narrative of my progress along the ancient Dyke, by which I passed so near to the venerable Castle of Chirk, I have hitherto purposely omitted an account of that celebrated mansion. I shall now, however, return to it. CHIRK CASTLE. This noble and ancient pile was built on the site of Castell Crogen, about the year 1011. I must here hazard a conjecture concerning this Castle. I am of opinion that the old Castle of Crogen was then enlarged and repaired, not entirely rebuilt; as John Myddleton, who communicated a paper to the Society of Antiquarians on the subject, says it was begun A.D. 1011, and was finished in 1013, which makes it only about two years, probably too short a period for the total re-erection of so large an edifice. The building is square, and is flanked by four massive bastions or rounders, one at each corner. There is a fifth in the centre of the front, of the same dimensions as the others. The length of the front is about two hundred and fifty feet; the square court or quadrangle within the walls is about one hundred and sixty-five feet, by one hundred. The grand entrance is under a lofty arched gateway; the side entrance is by a double flight of stone stairs, through a postern and colonnade, into the quadrangle. I measured the walls of the north-west bastion, and found them more than fourteen feet thick, and all apparently of solid masonry. The whole of this extensive and ponderous building is of hewn stone.