"A forge is a furnace where metal is heated or melted. In ancient times smiths with small forges could be found in most settlements. The furnaces were stoked with coal, which was fanned by manually pumped bellows to create intense heat. The blacksmith would heat the metal in the oven and then shape it by putting the metal on an anvil and beating it with heavy hammers. The finished product was then put into cold water to harden and cool. The smith might also melt the metal and pour it into molds.
Modern forges are huge industrial structures requiring many workers to operate. The operation is similar to that in the smithy; except on a grand scale. Tons of ore are poured into massive vats, which are conveyed into the furnace. Once the metal is smelted, it may be mixed with other materials to create an alloy such as steel, and it may undergo a number of different processes to remove impurities. The finished metal can then be rolled into sheets, poured into molds, stamped into coins, and so forth."
"The Forge allows a city to produce units and buildings faster, but also adds unhealthiness in cities. Forges are required to build the Colossus, Eiffel Tower, and Statue of LIberty world wonders, and the Ironworks national wonder."
In the city screen press the following icon to start Forge construction: