The original castle of Carisbrooke consists of a high motte, ditched round, placed at the corner of a parallelogram with rounded corners. This bailey, covering 2-3/4 acres, is surrounded by high banks, which testify to the former presence of a wooden stockade. There is another bailey on the eastern side, called the Tilt-yard. The excellent little local guide-book compiled by Mr Stone calls this a British camp, but there is no reason to believe that it was anything else than what it appears to be--a second bailey added as the castle grew in importance. On the motte is a shell of polygonal form, of rubble masonry, but having quoins of well-dressed ashlar. It is believed to be of the time of Henry I., since the author of the _Gesta Stephani_ states that Baldwin de Redvers, son of Richard de Redvers, to whom Henry granted the lordship of the Isle of Wight, had a castle there splendidly built of stone, defended by a strong fortification.[340] This would indicate that, besides the stone keep, stone walls were added to the earthworks of the Domesday castle. The keep is of peculiar interest, as it still retains the remains of the old arrangements in keeps of this style, though of much later date. The motte was opened in 1893, and was found to be composed of alternate layers of large and small chalk rubble.[341] Little attention has hitherto been paid to the construction of these Norman mottes, but other instances have been noted which show that they were often built with great care. The whole castle, including the Tilt-yard, was surrounded with an elaborate polygonal fortification in Elizabeth's reign, when the Spanish invasion was expected. The value of the manor of Alvington had increased at the time of the Survey, though the number of ploughs employed had actually decreased. This increase must have been owing to the erection of the castle, which provided security for trade and agriculture. Alvington was not the centre of a large soke in the Confessor's time, so it is unlikely that there was any fortification there in Saxon days.[342]