Many of the castles of this age were of great size. Instead of a single tower, they consisted of several towers, both round and square, united by walls, inclosing a space called a courtyard, the entrance to which was generally between two strong towers. The whole building was surrounded by a moat or ditch, across which a drawbridge led to the massive doors, which were covered with plates of iron, and in front of them, an iron portcullis--like a harrow, such as we see in the arms of the city of Westminster--was let down the rough, deep grooves in the stonework; whilst overhead projected a parapet, resting on corbels, with openings through which melted lead or hot water could be poured, or stones thrown on the heads of the assailants, who should attempt an entrance by forcing, or, as was the usual mode of attack, by setting fire to the door.[24] The gateways of Caerlaverock, Conway, Carisbrooke, and Caernarvon castles, present good specimens of this kind; as do the Middle Tower, and the Bloody Tower, in the Tower of London: the latter has the most perfect portcullis in the kingdom. A principal tower or keep rose prominently above the rest, and generally from an artificial mount. It contained the well of water, without which the garrison, when besieged, could not hold out in this their last place of refuge. The keep also had its subterranean prison, and several stories of apartments communicating by a staircase, either in the walls, or built outside the tower. As the railway traveller journeys along the South Eastern line, he will see close to the Tunbridge station, the towered entrance-gate of the castle built by Richard de Tonbridge, a follower of the Conqueror. The whole building was moated, and the exterior walls inclosed an area of about six acres. There remain only two massive towers flanking an arched gateway, with walls of great thickness, and having no other openings than long narrow slits, called _oilets_, through which, when besieged, archers shot their arrows. In front of this entrance was formerly a drawbridge, thrown across the moat, which, when raised, formed a strong door, closing up the archway. This opening was again guarded by two portcullises and two thick doors. The towers appear to have been divided into four stories, or floors, the lower being dungeons or prisons, and the upper formed into a large and noble hall, extending the whole width and depth of the two towers. It was lighted by two large windows towards the inner court. The towers are supposed, from their style, to have been built in the reign of King John, or Henry III. The windows were not glazed, but had iron bars; the floor and ceiling were of immense thickness, the latter three feet. Branching from this tower-entrance, are certain walls to the right and left; the first extending up the side of a lofty hill, whereon was the keep-tower, or chief residence of the baron: to this, it is presumed, he retreated when other parts of his