One of the most important developments in the history of warfare was the invention of firearms. Originally developed in the 14th century, firearms evolved quickly, and by the late 17th century flintlock muskets had become commonplace in battle. Musketeers, who were relatively inexpensive to equip and train, proved a deadly foe for the knights who had dominated the battlefield for so many years. Armies were often comprised of a combination of musketeers, cannon, and knights, each complementing the strengths and weaknesses of the others. Large forces of musketeers within a fortress or behind city walls proved an exceedingly difficult obstacle to an attacking army. Muskets were the primary firearms in use until they were replaced by repeating rifles in the mid-19th century.