The Catholic Church was the dominant institution in Europe during the Middle Ages and wielded significant political power. Bishops and other high-ranking clergy often sat on kings' councils and helped govern. Monasteries were run according to St. Benedict's rules, with monks and nuns taking vows of poverty, chastery, and obedience. Pilgrimages to holy sites were also an important part of religious life. The Crusades were a series of wars between Christians and Muslims over control of holy sites in Palestine. In response to heresy, the Inquisition was established in 1233 to investigate and punish those accused of holding beliefs contrary to Church doctrine.