Danger from without was accompanied by disorganization within. In the tenth century the political edifice so painfully constructed by Charlemagne was in ruins. The organization of the Roman Empire and the Gregorian ideal of a Catholic Church, now little more than a lingering tradition, was replaced by the feudal system. Seigneurs, lay and ecclesiastic, warring among themselves for the shadow of power, had neither time nor inclination for the ways of peace or the life of the spirit. Learning all but disappeared; the useful arts were little cultivated; cities fell into decay and the roads that bound them together were left in unrepair; the life of the time, barren alike in hovel and castle, was supported by the crude labor of a servile class. To be complete within itself, secure from military attack and economically self-supporting, were the essential needs which determined the structure of the great fiefs. The upper classes rarely went far afield, while the "rural population lived in a sort of chrysalis state, in immobility and isolation within each seigneury." But the feudal regime, well suited to a period of confusion, could not withstand the disintegrating effects of even the small measure of peace and prosperity which it secured. Increase in population and the necessities of life liberated those expansive social forces, in politics and industry, in intellectual life, in religious and emotional experience, which produced the civilization of the later Middle Ages; that wonderful thirteenth century which saw the rise of industry and the towns, the foundation of royal power in alliance with a moneyed class, the revival of intellectual activity which created the universities and the scholastic philosophy, the intensification of the religious spirit manifesting itself in such varied and perfect forms,--in the simple life of a St. Francis or the solemn splendor of a Gothic cathedral. Of this new and expanding life, the most striking external expression was embodied in the Crusades. Strangely compounded of religious enthusiasm and political ambition, of the redeless spirit of the knight-errant and the cool calculation of the commercial bandit, these half-military and half-migratory movements of the twelfth and thirteenth centuries mark the beginning of that return of the West upon the East which is so persistent a factor in all modern history. Christendom, so long isolated, now first broke the barriers that had closed it in, and once more extended its frontier into western Asia: Norman nobles, establishing the Kingdom of Jerusalem and the Latin Empire, enabled the Church to guard the Holy Sepulchre, while Italian cities reaped a rich harvest from the plunder of Constantinople and the Levantine trade.