I think that the achievements are fairly easy to get, but it's still a really fun game. Yeah, after you kill the main guy it's just grinding.. but it gives you a chance to finish off any quests or achievements you may still need to get. I played for hours just seeing how far up I could build my army.
@flyingpanda6 The rocks that have grey lines on them give you coal, the rocks with yellow lines give you gold, and the rocks with no lines give you rock.
The rocks that give you rock don't appear as often as the other types of rock.