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"You should hammer your iron when it is glowing hot."
-Publilius Syrus
Strategy
Iron Working reveals the Iron resource and allows Swordsmen to be trained (they require access to Iron.) It also allows your Workers to cut down jungles.
Civilopedia Entry
Some examples of iron ornamentation date back to 4000 BC, but the use of iron for tools, weapons, and other practical purposes did not become common until much later. Prior to this time, bronze was the most widely used metal for such purposes. Although the term "Iron Age" denotes a period of time starting around 1000 BC, iron replaced bronze as the metal of choice at different times in different places.
Iron is more common than copper and tin, the component metals of bronze, but iron is seldom found in a free state; it is most commonly found mixed with other minerals and elements. In order to be used, the iron must be separated from the ore. Once this technique was developed, and forges hot enough to melt the iron were developed, iron working became commonplace. Worked iron was harder, less brittle, and could hold a much sharper edge than bronze. Iron has remained an important metal throughout history, and is one of the major components in the production of steel.