It is difficult to form a decided opinion as to the age of the several works in masonry remaining in this castle. The two gateways and much of the curtain, especially its north and west sides, are probably early Norman. The keep and observatory tower are later in the same style. To judge from the little postern, the curtain between the observatory mount and the keep, and probably the part beyond it, are also late Norman; that is to say, the English defences of the keep were the last to be replaced, and it was a century before the isolation of its mound was broken by the carrying the curtain across its ditches, and the completion of its general _enceinte_. Altogether it seems probable that the lower stage of the two gateways, and the older part of the curtain, were constructed during the reign of the Conqueror, or, at any rate, before the close of the eleventh century. The keep and observatory tower were probably built, the upper floor of the gateways added, and the curtain raised and completed, in the reign of Stephen, who granted, with the castle and city of Lincoln, to Gernons, Earl of Chester, licence to fortify a tower in the castle, and to hold it until he recovered his own castle of Tickhill,—and even then, when he surrendered Lincoln, he was to retain his own tower, which his mother, Countess Lucia, had fortified, in the castle, of which also he was to retain the hereditary constableship. As the keep was the only part of the castle which could be held independently of the rest, it must be to it that the charter relates. It is curious that Tickhill should also have a mound and polygonal keep. As to the later works, Cobbe Hall, and the additions to the observatory tower and the eastern gateway, are probably the work of Thomas of Lancaster, Earl of Lincoln, who held the castle from 1312 to 1322. The Pipe Roll of 2 John, A.D. 1200, records a charge of £20 by the constable of Lincoln Castle for the repairs of the new tower, probably the keep.