observed, 'A mounted knight is irresistible; he would bore his way through the walls of Babylon.'
Their importance can be seen in the fact that the Normans brought trained war horses with them when they invaded England in 1066; by contrast their Viking predecessors in the ninth century had counted on capturing horses in East Anglia so that they could move more quickly. In Germany trained knights proved an effective response to Magyar attacks in the tenth century and Slav ones thereafter; so much so that the political structures which the Slavs had built around war bands had to be redeveloped on the western model. At the same time the ship-borne raids of the Vikings became increasingly ineffective as their potential victims evolved forces of knights to counter them. The Viking threat, still major during the tenth century, thus became less serious during the eleventh, being directed against societies which had not developed armies of knights ( England, Ireland, the Northern Isles, southern shores of the Baltic) or within Scandinavia.
Knights represented one answer to the threat of attack, and equally offered better prospects to the aggressor, but the period saw still more expensive developments in the construction of fortifications. A war-leader who held men to him by prospect of plunder still needed to provide safe places in which shrines could be built and cattle driven in the event of a crisis. In tenthcentury Poland these were provided by a score of massive log and earthenwork forts which the Polish duke garrisoned with paid soldiers. Further west fortifications tended to be a good deal more modest, though the Polish forts had some counterpart in the castles which Henry I of Germany and his successors constructed against Magyar, Danish, and Slav raids. The normal form was a wooden building and stockade, perhaps on a natural or artificial mound; but such structures were vulnerable to fire and we know that one of them was torn down by a priest and his parishioners, enraged at its lord's exactions from them. For those who could afford it the alternative was to build in stone, and there are examples of such castles in Anjou from the late tenth century. In Germany their construction came about a century later.
A lord thus found himself in an arms race with his neigh-
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