It appears, then, that in the four northern counties there are at least 103 strong places, of which ten boast rectangular Norman, and one or perhaps two, shell, keeps, while of ninety-one little is known. CHAPTER VIII. THE CASTLES OF ENGLAND AND WALES AT THE LATTER PART OF THE TWELFTH CENTURY (_concluded_). There remain to be enumerated the castles west of the Severn and the Dee up to the Dyke of Offa. To this tract must be added on the one hand the half of Shropshire, which was on the English side of the Severn; on the other, one or two valleys like those of the upper Severn and the Wye, penetrating into the heart of Wales; and to the north and south a tract of seaboard, reaching in the one case to the Conwy, and in the other to Pembroke and including Aberystwith. Of this border-land, divided between the northern, middle, and western Marches, the first was given in charge by the Conqueror to Hugh Earl of Chester; the second to Roger Earl of Shrewsbury, Arundel, and Montgomery; and opposite to the last, most of which was as yet unconquered, was placed William FitzOsbern, Earl of Hereford. In the north and middle Marches the opposing parts of Wales were mountainous and strong, exceedingly dangerous to invade, and of little value to a conqueror; but in the south the country was far more open and more fertile, far less dangerous to the invader, and offering far greater attraction to the cupidity of the settler. The general policy was to penetrate the country by the open valleys and the seaboard, and at certain frequent points to erect castles strong enough to resist a sudden attack, and occasionally capacious enough to contain men and stores sufficient to reinforce troops in the field, or to receive and rally them when worsted.