During the reign of Henry I (1100-1135) masonry began to take the place of earthwork defences, but in due time the need of stronger defences became apparent, and during the reign of Henry II (1154-1189) the keep, citadel, and defensive works to the north were carried out at the enormous expense of nearly £5,000. The keep, one of the most important of the new works, forms a striking feature of the castle. In plan it is practically square, measuring 98 feet by 96 feet, exclusive of the fore-building, with walls at the lowest stage no less than 24 feet in thickness. This is amongst the largest buildings of its class in this country. Each of its three floors, basement, and first and second storeys, is occupied by two large apartments, those on the second floor being the chief or state apartments and possessing two tiers of windows. Dover Castle suffered a siege in 1137, and again in 1216. The latter occurred under the second constableship of Hubert de Burgh at the hands of the Dauphin Louis of France. (See the section on the Cinque Ports, pp. 196-204.) After this siege Dover Castle was strengthened by the construction of an additional defensive work, commanding the plateau to the north of the castle, and other works, including a subterranean passage, excavated in the solid chalk, which still exists. These works were carried out between 1220 and 1239. In 1371 a series of important repairs was effected, and during the reign of Edward IV the Clopton tower was rebuilt, and a sum of £10,000 was expended in placing Dover Castle in a state of thorough repair. [Illustration: FIG. 29. BIRD'S-EYE VIEW OF DOVER TOWN AND HARBOUR, _temp._ QUEEN ELIZABETH] Further important works were carried out by Henry VIII in connection with his great scheme of coast defence. In addition to the strengthening of the actual works of the castle, it appears that "bulwarks under Dover Castle," probably near the level of the sea-shore, and a "bulwark in the cliff" were constructed at this period. An interesting plan of Dover, made in the time of Queen Elizabeth, shows not only the Arckcliffe Bulwark and the Black Bulwark, but also the walls and its towers inclosing the town of Dover. The plan was published in the sixth volume of "Archaeologia," and is here reproduced in much reduced size by permission of the Society of Antiquaries. In June 1666, and again in July 1667, an invasion of Dover by the Dutch fleet was expected. The invasion of this particular part of the sea-coast was never carried out, but the castle was provisioned for a siege, and it is probable that the actual fortifications were improved and augmented.