The larger class of private castles is less important from the institutional standpoint. In Normandy the duke had the right to garrison the castles of his nobility with troops of his own, but the Conqueror does not seem to have extended this principle to England. It is very probable that he would insist on his own consent being given to any projected fortification on the part of his feudatories, but so long as his rule was threatened by English revolt, rather than by Norman disloyalty, he would not be greatly concerned to limit the castle-building tendencies of his followers. On the Welsh border, for example, where the creation of a strong line of castles was an essential part of the business of frontier defence, the work of fortification must largely have been left to the discretion of the earls of Shrewsbury and Chester, and to the enterprise of the first generation of marcher lords. East of a line drawn north and south through Gloucester, lie nearly half of the total number of castles which we can infer to have been built during the Conqueror’s reign, but only fourteen of them were in private hands.
Underneath all these violent changes in the higher departments of the military art, the old native institution of the fyrd lived on. Two years after Hastings, at the dangerous crisis occasioned by the revolt of Exeter, we find the Conqueror calling out the local militia, and at intervals during his reign the national force continues to be summoned, not only by the king but by his lieutenants, such as Geoffrey of Coutances at the time of the relief of Montacute. It is not necessary to assume that William had prescience of a day when an English levy might be a useful counterbalance to a feudal host in rebellion; he inherited the military as well as the financial and judiciary powers of his kinsman King Edward, and obedience would naturally be paid to his summons by everybody who did not wish to be treated as a rebel on the spot. It does not seem that the Conqueror materially altered the constitution or equipment of the fyrd; in fact he had no need to do this, for its organisation and armament, obsolete as they were in comparison with those of the feudal army, still enabled it to fight with revolted Englishmen or Scotch raiders on more or less of an equality. For the serious business of a campaign the Conqueror would rely on the small but efficient force of knights at his command, and it is to be noted that no barrier of racial prejudice prevented the absorption of Englishmen of sufficient standing into the knightly class. The number of Englishmen who are entered in Domesday Book, on a level with the Norman tenants of a great baron, is considerable, and it is by no means improbable that, below the surface of our records, a process had been going on which had robbed the heterogeneous militia of King Edward’s day of its wealthier and more efficient elements. Many a thegn who would formerly have joined the muster of his shire with an equipment little, if at all, superior to that of the peasantry of his neighbourhood, will have received his land as the undertenant of some baron, and have learned to adopt the military methods of his Norman fellows. We cannot define with accuracy the stages by which this process did its work, but when the time came for Henry II. to reorganise the local militia, it was with a force of yeomen and burgesses that he had to deal.