The three principal fortresses which, placed upon English territory, formed in a military point of view the base for operations in Wales, were Chester on the Dee, Shrewsbury upon the Severn, and Gloucester at the mouth of the same river, the last being under the Mercian kings a place of great strength and importance. Chester and Gloucester were of Roman, Shrewsbury of British origin. In advance of Chester, and beyond the estuary of the Dee, were the castles of Hawarden, Ewloe, Halkin, Flint, Diserth, Rhuddlan, and Diganwy, extending upon or at no great distance from the sea coast as far as the Conwy river. More inland and south of these are Caergwrle, Mold, Ruthyn, Denbigh, and Basingwerk, the last nearly upon Watt’s Dyke. Hope Castle is mentioned, but is probably the same with Caergwrle, and Overton, which, though on the Cheshire side of the Dee, was held by the Princes of Powis. It seems to have been founded by William Peverel, and was by him defended against Stephen in 1138. Most of these castles are mere ruins; of others the extant buildings are Edwardian additions. Hawarden, however, has a tolerably perfect circular keep, with a small mural oratory, and an exceedingly complex but later arrangement for defending the approach. Considerably south of these castles, where the Dee has ceased to be the dividing stream of England and Wales, is the hill castle of Dinas Brân, an early and strong place, by whom built is uncertain, but which was held for long periods by the Welsh Princes. Again, south, upon the Ceiriog river, and a few yards east of Offa’s Dyke, is Chirk Castle, inhabited and much altered, but of early date. The western side of Shropshire and the adjacent part of Montgomery formed nearly the whole of the Middle March. Earl Roger, its custos under the Conqueror with powers which William only delegated from absolute necessity, and which it took his successors centuries to resume, was the only Norman lord who gave name to a county in the conquered land. Under him and his lieutenants, Roger Corbet and Roger Mortimer, the March was feued out to a number of knights and lesser barons, all of whom built castles, and thus defended the common territory, while protecting their own private estates. The task of construction probably was not so onerous as at first sight might appear. Shropshire and Herefordshire, and especially their most exposed parts, were already studded over thickly with strong places constructed after their own fashion by the Mercian invaders and settlers, and of these it is evident that the new lords availed themselves until they were able to make additions to them in masonry.