| In medieval times, mercenary companies were extremely common. These soldiers would be hired as a complete unit, and fielded as such. The Genoese crossbowmen at the Battle of Crecy were such, as were the late-medieval Italian condotteri (literally "contractors"). Other examples of medieval mercenary companies included the Varangian Guard of Constantiople and so-called free-lancers: mounted horsemen with no fealty to a specific lord, literally a "free lance" who could be hired into service.
In period they were used to supplement (or substitute for) a kingdom's army -- their main drawback lay in the fact that, once the principle of fighting for money had been established, it was not long before the concept came along of changing sides in return for more money. Mercenaries value was increased by the fact that the feudal rule of "forty days' military service" did not apply to them: they would be obliged to fight as long as they continued to be paid.