During the 500-1000 CE period known as the "Dark Ages", Europe experienced invasions and the destruction of centers of learning, leading the Church to undergo reforms and massive building projects like cathedrals. Major issues the reformers focused on included priests marrying, selling church positions (simony), and lay investiture. In the 1100s-1200s, the Church restructured itself with the Pope at its head and established canon law. New religious orders like the Dominicans and Franciscans were also formed during this time.