New Economic and Urban Vigor. During the 9th and 10th centuries new agricultural techniques - the moldboard plow, the three-field system - significantly increased production. , Horse collars - also useful for agriculture - and stirrups confirmed lordly dominance. Viking incursions diminished as the raiders seized territorial control or regional governments became stronger. Both factors allowed population growth and encouraged economic innovation. Expanding towns emerged as regional trade centers with a merchant class and craft production. The need for more food led to colonization developing new agricultural land. The demand for labor resulted in less harsh conditions for serfs. The growing urban centers increased the spread of literacy, revitalized popular culture, and stimulated religious life. By the 11th century cathedral schools evolved into universities. Students studied medicine and law; later theology and philosophy became important disciplines. Art and architecture reached new peaks.