Gunpowder allows the building of musketeers, the staple of defense for the Middle Ages.
It is a prerequisite of explosives and metallurgy.
It negates the effects of the Great Wall, replaces militia and phalanx units and obsoletes existing barracks.
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Civilopedia
Gunpowder is a chemical mixture of saltpeter, sulphur, and charcoal. When ignited, it burns so rapidly that it explodes if contained to any degree. Gunpowder is believed to have been developed by the Chinese as early as the 9th Century, but did not reach Europe until the 1300s. The Chinese appear to have used it almost exclusively for fireworks, but the competing and aggressive Europeans turned it into weapons that revolutionized warfare. Musketeers and Cannon brought an end to the battlefield dominance of heavy Cavalry and also ended recurring invasions of barbarians from Asia.