The Goblins provide your typical weak but fast army with the Dwarves being the slow but strong tank style race, my personal favourite is the Elven army though as they seem to have the best balance. One of the changes from the previous game is the fact that you no longer take your soldiers with you to the next level and while this means that you don't have a unbeatable army within a few levels, it also means that most of the time you just cant be bothered to upgrade and level your basic army because you have to do it all over again the next map, so most of the time you end up just using your hero like you would in an RPG, not necessarily what the developers hoped for I think.