The Crusades were necessary. The Seljuk Turks would only let muslims into Israel, when the christians and jews wanted to get into the Holy Land, too. The christians needed hope that they could travel to Israel and practice their religion, so Pope Urban II declared a Holy War on the Turks. The crusades were huge failures, but it was still necessary because the Holy Land is extremely important to christians and jews, not just muslims.
The Feudal system was necessary. The Vikings were