Taking London as the centre, military and political, of the kingdom, we have, upon the Thames, the Tower, the first and chief fortress founded by the Conqueror, and which he considered sufficient to protect and overawe the city. In the city itself, also upon the banks of the Thames, near the outlet of the Flete, was Baynard’s Castle, the stronghold of the Barons Fitz-Walter, standard-bearers to the City of London, and an important branch of the House of Clare. At various distances from this centre, according to the disposition of the ground, were posted within the northern and southern passes of the chalk ridge, Berkhampstead, an appanage of the earldom of Cornwall, and Guildford, the early keep of which stands in part upon an artificial mound. Also, to the immediate south of London, were the episcopal castle, still inhabited, of Farnham, and Earl Warenne’s castle on the hill at Reigate, of which some traces remain. Higher up the Thames were Windsor, Reading, Wallingford, and Oxford, all fortresses of high antiquity and of the first rank. Between the Thames and the sea-coast the country was well guarded, and the communications with Dover, Portsmouth, and Southampton, so important to sovereigns with possessions on the Continent, rendered secure. Dover, called by William the Conqueror, according to Matthew Paris, “Clavis et repagulum,” the key and bolt of the kingdom, was one of its oldest, largest, and strongest fortresses, and covered a nearly impregnable area of thirty-five acres. It crowned the crest of a chalk rock which seemed to rise out of the sea, and steep by nature was rendered still more so by art, and bore traces of Norman, English, Roman, and probably British occupation. Its well of water is particularly specified, according to M. Paris, in Harold’s celebrated covenant with Duke William. Indeed, there seem to have been two wells in the keep, besides another, no doubt that of Harold, in the outer ward, probably a Roman work. The town also was walled. In the rear of Dover lay the city of Canterbury, mentioned in “Domesday” as fortified. It was strong to the landward, with a formidable bank and ditch, revetted by a Norman wall, and towards the water was covered by the marshes of the Stour, at one time navigable up to the quays of the ancient city. At one angle and just within the area, was a strong rectangular keep, a Norman addition, and near it was the Danejohn, a far older moated mound, older even than the bank and ditch of the city, which were laid out at an angle to include it. Near to Canterbury was Chilham, a Norman tower of peculiar form, on the site of a work burned by the Danes in 838-51; and at no great distance was Saltwood, given to the see of Canterbury in 1036, and said to owe the formidable banks and ditches which still surround it to a son of Hengist. West of Dover William d’Abrincis had built the castle of Folkestone, now, with the cliff it stood upon, swallowed up by the sea.