Mercenaries had become more common in the medieval age than regular soldiers. Based on the fact that mercenaries already had fighting skills and could be quickly deployed into battle. Thus no need to train them. A downside is that they weren't very trustworthy, they tended to only fight for the highest bidder. Reliable mercenaries such as the Swiss pikemen (imperial guard) were very popular in the late middleage. Not only for being very reliable but also for their unique formation called the pike square formation, and their unique, aggressive tactics.