There aren't that many new construction materials invented during the ages - straw, wood, stone, marble, steel... But this last one proves more useful for a wider range of things than any of the others, seeing how it little by little turns into the basis of modern-day construction of everything from ships to buildings. Thanks to steel, two new very important units become available - the Artillery on land, and the Battleship in the sea, which is truly the first modern ship. Steel also immediately finds its way into civilian construction, and improves the workings of the Lumber Mill.