The number and strength of the fortified places of Western Europe explain the apparent futility of many campaigns of the period. A land could not be conquered with rapidity when every district was guarded by three or four castles and walled towns, which would each need several months’ siege before they could be reduced. Campaigns tended to become either plundering raids, which left the strongholds alone, or to be occupied in the prolonged blockade of a single fortified place. The invention of gunpowder was the first advantage thrown on the side of the attack for three centuries. Even cannon, however, were at the period of their invention, and for long years afterward, of very little practical importance. The taking of Constantinople by Mahomet II is perhaps the first event of European importance in which the power of artillery played the leading part. Before proceeding to discuss the rise of the new forms of military efficiency which brought about the end of the supremacy of feudal cavalry, it may be well to cast a glance at those curious military episodes, the Crusades. Considering their extraordinary and abnormal nature, more results might have been expected to follow them than can in fact be traced. When opposed by a system of tactics to which they were unaccustomed the Western nobles were invariably disconcerted. At fights such as Dorylæum they were only preserved from disaster by their indomitable energy: tactically beaten they extricated themselves by sheer hard fighting. On fairly-disputed fields, such as that of Antioch, they asserted the same superiority over Oriental horsemen which the Byzantine had previously enjoyed. But after a short experience of Western tactics the Turks and Saracens foreswore the battlefield. They normally acted in great bodies of light cavalry, moving rapidly from point to point, and cutting off convoys or attacking detached parties. The Crusaders were seldom indulged in the twelfth century with those pitched battles for which they craved. The Mahometan leaders would only fight when they had placed all the advantages on their own side; normally they declined the contest. In the East, just as in Europe, the war was one of sieges: armies numbered by the hundred thousand were arrested before the walls of a second-class fortress such as Acre, and in despair at reducing it by their operations, had to resort to the lengthy process of starving out the garrison. On the other hand nothing but the ascendancy enjoyed by the defensive could have protracted the existence of the ‘Kingdom of Jerusalem,’ when it had sunk to a chain of isolated fortresses, dotting the shore of the Levant from Alexandretta to Acre and Jaffa. If we can point to any modifications introduced into European warfare by the Eastern experience of the Crusaders, they are not of any great importance. Greek fire, if its composition was really ascertained, would seem to have had very little use in the West: the horse-bowman,