Ironically the ability of castles to withstand a prolonged siege was one of the factors that led ultimately to their downfall. Under the feudal system, a knight was obliged to fight for his lord only forty days at a stretch, but a well-built and well-provisioned castle might hold out for six months or a year. Obviously, a lord who was in the midst of a siege could not depend on soldiers who, when their tour of duty was up, could just ride off. So, over time, knights who were bound to a lord by nothing but loyalty began to play a smaller and smaller role in warfare. At the same time, the role played by mercenaries grew ever larger. Increasingly, as Jay Williams explains,

the real fate of the battle rested on the pikemen, the archers, and finally the gunners...