Usually to a town or city. Surviving in a city "for a year and a day" (the time varied by region) would render the serf free. Some former serfs prospered in this way, others simply became part of the urban underclass. In any event, they were no longer serfs. Other sefs ran away to become mercenary soldiers (often by first serving as servants for the soldiers), sailors or brigands. The Crusades (from 1095 to 1400) were a tremendous excuse for a serf to leave his manor and make his way in the world. The growth of towns, clergy and undertaking crusades were all a result of the increased agricultural productivity.