Depending on where and when you're talking about, if you were a man, if you weren't farming, you were at the front lines of the king's army with a spear and no armor.
That was actually fairly uncommon for most of the middle ages. From the collapse of the western empire until the military reforms c1500, standing armies were few and far between. Peasants could be drafted to fight by their lords, but time in military service was the exception rather than the rule.