Iron is obtained by building a Mine improvement on the resource. A mined Iron tile provides a total of +3 to the tile, and an unmined Iron tile provides a +1 to the tile.
The resource (once the Iron Working technology has been researched) provides the Iron Weapons promotion to all Melee units (with the exception of the Warrior which can only equip Bronze Weapons ) as well as some Mounted units (e.g. Chariot). Once iron is connected, to gain the promotion, a valid unit must first visit a city connected to the Iron resource to gain the promotion.