The Romans, who always had a keen eye for favourable defensive sites, were scarcely likely to miss the high ground in the great bend of the Medway not far from where it falls into the Thames. The Watling Street from Dover to London passed the river, moreover, at this point and received protection from the Roman camp. The Saxons and Danes also maintained the Castle of Hrofa here. The usual timber fortifications were constructed in an oblong enclosure of about seven acres, including a large conical mound of the Eastern chalk range called Bully Hill. It must even at that date have been a place of some strength, because when it was besieged by the Danes in 885, it was able to hold out long enough for Alfred to come to its relief. At the Conquest, the Normans recognized the strength of the position and added their own improved methods of fortification, enclosing a quadrangular space close to the river with a strong curtain wall and afterwards building a massive square keep in the enclosure. The Saxon works were left outside and used merely as an outpost, as was the case at Warwick and Canterbury. The original quadrangular enclosure had a wall-circuit of 580 yards, the North and South walls measuring 160, and the East and West 130 each. The East front faced the cathedral which even at that day was venerable. The West wall ran along the river front. The other three walls were defended by a broad and deep moat, traces of which still remain. Much of the outer wall, with square open towers recurring at intervals also exists. The main entrance or gatehouse with drawbridge, which no longer exists, was at the North-east angle, from which there was a steep descent to what is now the High Street. At the North-west angle, was a bastion tower with a postern gate. Although this tower no longer exists, it was still standing as late as 1735, immediately on the shore, commanding the bridge. A large round tower still stands at the South-east angle. It measures thirty feet in diameter; it has two floors and is loop-holed for archery. Two rectangular towers that defended the East front are still in existence. Throughout the constructions, we cannot fail to notice and admire the strength and massiveness of the masonry. To this the ruin owes its preservation, for besides the destroying hand of time, the neglect of unappreciative generations, and the destruction wrought by greed and fanaticism, it has also suffered from several sieges.

On the highest ground of the enclosure, near the South-east angle, stands the keep. In grandeur and impressiveness, this tower does not suffer in comparison with any English keep of the Norman period. Neither the smaller keep of Newcastle, nor the larger ones of Colchester, Canterbury, Norwich and Hedingham show the original arrangement better than Rochester. Its base is more than seventy feet square, and it is 113 feet high. It is buttressed at the angles with four small towers each twelve feet square. These, rising twelve feet above the principal mass, add greatly to the picturesque effect of the whole. Clinging like a limpet to the East angle of the keep is a smaller tower twenty-eight feet square and about seventy-five feet high. This contained the chief entrance to the keep. It had a flight of steps and an arched gateway enriched. This and the other arches are constructed of Caen stone brought from Normandy; the walls, from ten to twelve feet thick, are built with Kentish rag. Even when this smaller tower was taken in an assault, the besiegers still had trouble to get into the keep proper, for the vestibule was divided from the rooms of the great keep by a portcullis in the main wall. The groove in which it worked and traces of the ironwork still remain.

The keep contains three storeys of lofty apartments, with a vault beneath. As in the tower of London, it is divided into two nearly equal parts by a wall running East and West that rises to the roof. Its thickness allows it to contain a well two feet nine inches in diameter with openings by which each apartment might be supplied with water. By this arrangement, it was impossible for besiegers to cut off the drinking supply of the garrison. The thickness of the walls also admits of mural galleries, as in the White Tower, and a well staircase leading from the vault to the roof, communicating with each apartment. The basement and first floor received their light through loop-holes; the rooms of the higher storeys have their walls pierced with windows.

On the second storey were the rooms of state, thirty-two feet high. It has two tiers of windows, the upper tier having a passage in the wall in front of the windows. On this floor, the apartments open into one another through the central dividing wall by four arches; and in the north-east corner is a large doorway leading into an oratory or chapel built over the great entrance. A flight of steps ascends to the wall gallery which goes all round the tower: as in the White Tower, it is vaulted. It is three feet high. In these apartments, there are fireplaces with enriched arches from which the smoke escaped through openings in the wall near the hearth. This primitive contrivance must have rendered the council-chamber and banqueting-hall uncomfortable with draughts.

Twenty-three steps lead from the wall gallery to the top floor which contains two handsome rooms twenty-five feet high. From this storey, the visitor may enjoy a lovely view, including the town and banks of the winding river, the Cathedral and its close, extending in the distance to the junction of the Thames and Medway.

Above the third floor, are the battlements which had a rampart walk. The floorings were all carried by timber joists, and in the basement was a prison.

The striking resemblance between this keep and the White Tower at London of the same date would lead us to conclude that both were designed by the same architect. They were in fact both planned originally by Gundulf, who was consecrated Bishop of Rochester in the year after the Conquest. Besides his other great attainments, this bishop was a very able architect, and when the Conqueror wanted to erect a strong castle at Rochester, Gundulf was naturally entrusted with the task.

The first important historical event connected with the castle was the rebellion of Odo, Bishop of Bayeux and Earl of Kent, half (and perhaps full) brother to William the Conqueror. Kent had already suffered greatly from his rapacity, and his conduct finally led to his dramatic arrest by William’s own hands. After William’s death, he plotted in Robert’s interest against his nephew Rufus. He attributed his imprisonment to Archbishop Lanfranc, and when war broke out between the brothers Robert and William in 1088, he plundered Kent, paying especial attention to the Archbishop’s estates. Finally, being captured at Pevensey, he was forced to give up all his possessions in England, including Rochester, and leave the country. He was sent under guard to Rochester to complete the surrender and take ship for Normandy; but on his arrival, Eustace of Boulogne and Robert de Belême and other supporters rescued Odo and refused to surrender the city. The castle was garrisoned and William Rufus besieged it in person. It surrendered after a blockade of six weeks. William was very reluctant to grant any terms, and indignantly refused Odo’s request for the honours of war. The English portion of William’s army, who were principally Kentishmen, were very bitter against the Bishop who had harried and oppressed them, and cried: “Halters! halters for the traitor bishop! Let not the doer of evil go unharmed!” Counsels of clemency, however, prevailed; and Odo was allowed to go; and on this occasion Rochester saw the last of him.

The castle had been considerably injured in the siege, and William commissioned Gundulf to spend £60, a large sum in those days, in building a new tower.

In the twenty-seventh year of William’s successor, Henry, the king, with the consent of his barons, granted to the church of Canterbury, William (of Corbeil), archbishop of that see, the custody of the castle of Rochester for ever, with liberty to build a fort and a tower. This archbishop, who had the support of the king in the rivalry of Canterbury and York, was a great builder. He rebuilt Rochester Cathedral and attended its dedication in 1130. Shortly before, he had with great pomp completed and dedicated the great cathedral at Canterbury begun by Lanfranc. It was therefore about 1130 that the new castle was also completed.

The castle with its splendid and strong keep was far too important a military post to remain in possession of the see of Canterbury for any length of time in that turbulent age. When the see became vacant, and on other occasions, the Crown resumed possession of it. In 1141, William of Ypres, a Fleming, was its governor for Stephen, as the archbishop had sworn allegiance to the Empress Maud. When the Earl of Gloucester, a natural son of Henry I. was captured at Winchester, he was imprisoned in this castle until exchanged for Stephen, who was taken at Lincoln later in the year. William of Ypres being banished, Henry II. gave his earldom of Kent and the castle of Rochester to Philip, Earl of Flanders, but the Earl never took possession.

In 1202, the castle was again restored to the archbishop, then Stephen Langton, who later, during John’s wars with his barons, turned it over to William de Albini, a valiant and able commander, to be held in the interests of the barons. John invested the stronghold in 1215, and succeeded in gaining possession after an obstinate defence lasting three months. The military engines could produce little impression, but the walls were undermined, and then the keep was attacked in the same way. The following year, Louis the Dauphin, being invited over by the barons to assist them against John, landed at Sandwich and led his army to Rochester. The damage had not yet been repaired and so the castle easily fell. With other Crown possessions, it then came into the hands of Henry III. Much money was spent in repairs, especially in 1225–6–7. This was while Hubert de Burgh was constable of Rochester castle. In 1240, the tower was ordered to be whitewashed where it had not yet been done; and in 1247 both chapels were ordered to be wainscoted. One of these was in the outer ward, and used by the garrison.

In 1264, the king gave the charge of the castle to the celebrated Roger de Leybourne who had just joined his cause. He furnished it with sufficient arms, garrison, and provisions to stand a siege. Early in April, the attack being imminent, the king’s brother-in-law, the Earl of Surrey, arrived at the castle with reinforcements. Just before Easter, Simon de Montfort came to besiege the castle. On reaching the western bank of the Medway, he found the passage of the bridge disputed, and a palisade and breastwork thrown up on the opposite side, well defended. Having sent Gilbert de Clare to attack the south side of the town, the Earl of Leicester in person assaulted the bridge, but was twice driven back by the citizens. At length, with the aid of boats loaded with combustibles, he set fire to the bridge and the tower upon it which were both built of wood. During the confusion caused by the fire, he crossed the river and destroyed the church and what was left of the priory. Richard de Leybourne for purposes of defence had already burned down all the suburbs and part of the priory. Simon de Montfort next made a furious assault upon the castle and captured the outworks and all the towers except the great keep. The latter made such an obstinate resistance that after a seven days’ close siege, Simon suddenly relinquished the attempt and retreated to London. Shortly afterwards, in 1264, most of the garrison, under Leybourne, who had been badly wounded, left Rochester and joined the Royal army at Lewes. The king’s disastrous defeat there resulted in the surrender of Rochester castle to the Baron’s forces. When however the tide of success turned two years later, on the death of de Montfort at Evesham, and the fall of Kenilworth, Leybourne resumed his governorship.

In 1274, Robert de Hougham died constable of this castle, and was followed by Robert de Septvans. Two other constables of Rochester during this reign were Sir John de Cobham and Stephen de Dene. During the next two centuries the following names occur among the holders of this office: William Skarlett, Lord Grey of Codnor, John de Newtrun, William Criol and Sir Thomas Cobham.

In 1367–8, extensive repairs were undertaken by Edward III., under Prior John of Rochester as chief of the works. Stone was imported from Beer, Caen, and Reigate, with copings and crests for battlements, probably for buildings in the court. Edward IV. also repaired the castle, but afterwards it lost its military importance and fell into decay. A drawing, of the year 1518, shows the turrets domed over and capped with vanes, like those of the White Tower.

Rochester much resembles Hedingham, which is a very perfect Norman keep with three floors, the remains of a forebuilding and upper gallery in the main floor. In each ornamentation, the chevron moulding is profusely employed.