Barn/1.03

Empire
The Barn produces Elephants and that's all. Although Elephants are incredibly tough and deal high damage, their attack type isn't anything new for the Empire and the sheer cost of stone and metal to produce them severely hinders tiering. So you may want to leave the Barn till later, once you reach a Palace level 5 and your metal and stone is not as much in demand. Even then, their cost still greatly restricts their production and so building multiple Barns is a bit of a waste unless you have plenty of resources to spare.