In the Rise and Fall expansion, the Pikeman can upgrade to a Pike and Shot. In the Gathering Storm expansion, its Production cost is decreased from 200 to 180, its Gold purchasing cost is decreased from 800 to 720, and its Gold maintenance cost is decreased from 3 to 2.
Come the Medieval Era, civilizations with access to Iron can start fielding Knights, whose great mobility and Combat Strength can be devastating on the battlefield. Pikemen are your best defense against them, and have the advantage of not requiring any resources to produce. Use their long pikes to keep enemy Knights at bay, and provide a decent line of defense even if you have no Iron. Try to avoid having them fight Men-At-Arms, however, until they earn Thrust and Schiltron - Pikemen with these Promotions are a force to be reckoned with, but without them, the +5 Combat Strength bonus Men-At-Arms receive against anti-cavalry units will turn the tide in their favor.
Like the Spearmen from which they upgrade, Pikemen serve as the guards of barbarian outposts, making them difficult for Knights to capture and allowing them to withstand a few attacks from stronger cavalry units.
A pike is, basically, a very long spear, used for thrusting the sharp point into someone instead of throwing it at them. Very effective against other foot soldiers and especially against horsemen of all types. Indeed, besides longbowmen and muddy ground, the pike was about the only thing that would dissuade mounted knights from riding the peasants down. Thus, the pike was best used in mass formations, as by the Greek phalanx or Swiss mercenaries or German landsknechts. However, as it’s difficult to hold a pike with two hands and a shield simultaneously, pikemen tended to be vulnerable to archers and swordsmen; they weren’t much good in a melee. But if it could maintain close ranks, a pike-armed mass could roll right over the opposition, and could anchor the flanks against knights. During the heyday of its use in the Renaissance Era, the pikes became longer and longer, eventually some being 22 feet or so.