The population of Europe almost doubled between 1000-1300 AD due to increased stability, improved farming techniques, and favorable climate conditions. More land was cleared for agriculture. The introduction of iron tools like the plow and wind/water power aided farming. Most peasants became serfs legally bound to work lands owned by noble lords in exchange for protection. Serfs had many restrictions but could not be sold as slaves. Peasant life was simple and centered around agricultural seasons and numerous Catholic holidays. Emerging trade and the growth of cities in the 11th-12th centuries began shifting Europe's economic foundation away from being exclusively agricultural.