Of what character were those mentioned in “Domesday”? That William ordered many castles to be constructed is certain; and among the orders left with Bishop Odo and William Fitz Osborn, when acting as joint regents of the kingdom, was one specially charging them to see to the building of castles; and no doubt these orders were obeyed. But it has been hastily assumed that the castles constructed were of masonry. The keeps of Dover and Rochester, for example (if such were erected under the Conqueror), were certainly not those now standing, which belong to the reign of Henry II.; and so of Norwich, and probably of Nottingham, now destroyed. And yet the masonry of William’s reign was of a very durable character, as is seen in the Tower of London, and in not a few still standing churches. Also it is stated that William “custodes in castellis strenuos viros collocavit, ex Gallis traductos, quorum fidei pariter ac virtuti credebat.” This looks very much as though the castellans were at first, at any rate, put in charge of existing castles; which must mean that in most cases some temporary arrangement was made, and the existing works strengthened until it was convenient to replace them by others more in accordance with the new ideas of strength and security. William and his barons evidently employed two classes of castles,—one always in masonry, and one very often in timber. Where a castle was built in a new position, as in London, or where there was no mound, natural or artificial they employed masonry and chose as a rule for the keep the rectangular form,—a type said to have been introduced from Maine, and seen at Arques, at Caen, and at Falaise; but where the site was old, and there was a mound, as at Lincoln, Huntingdon, Rockingham, Wallingford, or York, they seem to have been content to repair the existing works, usually of timber only, and to have postponed the replacing them with a regular shell till a more convenient season, which in many cases did not occur for a century.