The additions directed by the Conqueror to the defences of the hill, already strong by nature and by art, rendered Lincoln, under the Norman dynasty, even a more important city than it had been under the earlier governments. Its castle was the almost impregnable fortress, held by or for the sovereign, of a very important division of England; but it was a division strong in its rivers and marshy ground, in its English and thoroughly disaffected feeling, and open to the visits of the Danes, no longer as enemies, but as allies to the cause of the people. Its position, dominating the whole shire, challenged comparison with Belvoir, which received a similar accession of strength, and Nottingham, on the brow of which a rectangular keep of the first class was then in progress; but what confirmed its central authority, and placed it far above any castled eminence of the counties of the Midland, was the recognition of the hill as the centre of an important bishopric, and the foundation by Remigius of the stately pile to which many succeeding centuries have added beauty and grandeur. The castle long remained a part of the demesne of the Crown but was administered by constables, whose office was, at times, regarded as hereditary, and, on one very important occasion, was held by a lady. Always a strong position, it became especially valuable upon the death of Henry I., when the long civil war broke out between his daughter and his nephew, and, by one party or the other, all existing castles were strengthened and an immense number of new ones built. In such a state of anarchy a castle became a necessary of life, and the bishops vied with, and even surpassed, the lay barons in their examples of military architecture. Sherborne, Malmesbury, and the strong and magnificent Devizes were the work of Bishop Roger of Salisbury, as were Newark and Sleaford of his nephew, Alexander of Lincoln; and the castle of Ely, of Giles, another nephew, and prelate of that see. Durham, also, was held by its bishop, and the Close of Lichfield strongly intrenched. Of the lay adherents of Matilda, Robert Earl of Gloucester, her wise and faithful brother, built the castles of Cardiff, Bristol, and Gloucester; Fitz-Alan held Shrewsbury; D’Albini, Arundel; Talbot, Hereford; Paganel, Ludlow; Brian Fitz-Count, Wallingford; D’Oyley, Oxford; Robert of Lincoln, Wareham; Mohun, Dunster; Lovel, Castle Cary; Mandeville, Walden and Plessis; and Fitz-John, Melton. Dover, much strengthened by Maminot, was surrendered to the queen. It is curious that, of all these castles, six only, Sherborne, Bristol, Ludlow, Walden, and Dover, with Hedingham, held by De Vere for Stephen, are certainly known to have had square keeps; of the others, seven are doubtful, but thirteen had shell keeps upon mounds.