The reign of Charlemagne, which was one long series of expeditions and conquests, was naturally favourable to the progress and development of this art. The Emperor of the Franks, like a man of genius, understood how to profit by the inventions and creations of his predecessors. To the warlike traditions of Greece and Rome he added, step by step, the improvements that were rendered necessary by the nature of the enemies with whom he had to contend, namely, the Lombards, the Saxons, &c. He kept up the feudal service of the _ban_; he established permanent orders of militia, composed of his own serfs and vassals; but, as soon as he undertook a distant expedition, his auxiliaries, ten times as numerous as his vassals, rendered his army rather a German than a French one. He caused a number of fortresses to be constructed everywhere throughout his vast empire, but he never allowed his subjects to build any on their own account. Yet he never seems to have attached any importance, as a protection to his territory, to the larger enclosed towns, in which he might have held in reserve considerable depôts of troops. He himself usually resided in rural residences and in open and unprotected villages, barely guarded by a few military pickets. At the slightest signal, it is true, a whole army of _fidèles_ and servitors would have arisen as one man to defend him; but under no circumstances would he have consented to await his enemy under the shelter of fortifications; he was always the true primeval German, seeking for his field of battle the open plain rather than the hillside, preferring cavalry to infantry, and a direct struggle, a hand-to-hand fight, to encounters at a distance, waged and won by the missiles of the slinger and the shafts of the bowmen. His principal victories were gained in the open country, where he was enabled to deploy his masses of mailed horsemen; he never willingly sat down before a stronghold, a circumstance which shows that he was aware of his want of skill in the conduct of a siege; and he was never fortunate in mountain warfare, as was evidenced in the disastrous day of Roncevaux (778), which cast a shadow over the last years of his life. Thirty years after the death of the great emperor, the treaty of Mersen (847) freed the great vassals from the obligation of answering the summonses of the sovereign, and of rushing to arms at his appeal, unless for the purpose of defending the State, and substituted the practice of furnishing armed contingents, whose services were to be rendered for a fixed period, settled beforehand. Infeudation--a kind of political and pecuniary contract, in virtue of which a fief was subdivided into several smaller ones--perpetuated the feudal _régime_, each man becoming _the man_ of another man, bound to place himself at his disposal in time of war, and to be ready to start on any expedition at his command, and according to the wishes of his immediate seignior.