Iron is one of the main resources in which you use to keep your settlement growing. The main use of Iron is to construct buildings and soldiers. Iron can be only accessed at the start of the Classical Age and is mostly used to build more advanced types of soldiers and buildings.
Iron can be obtained by constructing an Iron Mine on an empty Mountains tile within your settlement. Additionally, it can be recieved as a trade from other players, or another one of your settlements by using a Trading Post. You may also recieve 100 iron from the random resources each one of your Wonders create daily and some tasks offer iron as a reward as well. Another way to receive iron is by raiding/capturing iron, Precious Gems and/or Silver.