Even though the Middle Ages were tough, this era saw the rise of great military and political leaders, as well as amazing artistic, poetic, and architectural achievement. Charlemagne was the greatest ruler of the Middle Ages. The collapse of Charlemagne's empire and invasions led to a restructuring of medieval society. The 11th–13th centuries mark the high point of medieval civilization. The church underwent reform that strengthened the place of the pope in church and society but led to clashes between the pope and emperor. Population growth, the flourishing of towns and farms, the emergence of merchant classes, and the development of governmental bureaucracies were part of cultural and economic revival during this period.
Thousands of knights followed the call of the church to join the Crusades. Medieval civilization reached its peak in the 13th century with the rise of Gothic architecture, the appearance of new religious orders, and the expansion of learning and the university. The church dominated intellectual life, producing the Scholasticism of St. Thomas Aquinas. The decline of the Middle Ages resulted from the breakdown of medieval national governments, the great papal schism, the critique of medieval theology and philosophy, and economic and population collapse brought on by famine and disease.