The peasant. I'm pretty sure, that pound for pound, the peasant is the most efficient unit in the game.
For a measly 110 florins (?) you get a unit that can take out almost all spear units in the game, kill cavalry once they are entangled, and fill a variety of other purposes.
God forbid you try and fight peasants with mercenary spearmen if the peasants have one chevron of experience. I couldn't even imagine the slaughter.
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