A Civilization advance.
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 ninth century, but it did not reach Europe until the 1300s. The Chinese appear to have used it almost exclusively for fireworks, but the competitive and aggressive Europeans turned it into weapons that revolutionized warfare.
Prerequisites: Invention and Iron Working.
Allows for: Explosives and Metallurgy.