Where a population had a large percentage of non-slaves, the government usually made provision to mobilize the free men for military service. Citizens always available on standby in emergencies were called the levy. The ruler could call out the levy under certain conditions, but especially if enemies were approaching. The levy was a formal understanding and obligation, roughly equivalent to a tax or mandatory public service. Greek freemen, for example, from early adulthood to their 60’s were expected to serve in their town’s phalanx, a local variation of the levy.