I admit that the artwork could be better, but this is really a good game. I think it's because you figure out how to play it as you go along. I didn't even realize that you could use the boulders to kill the monsters throwing them. They deserve it! Their little grunts are funny too.
While not too complicated, it's harder than it looks. I like to see how the buildings withstand damage. I get the feeling this game just goes on forever. Not to say that doesn't make it good. Redundant titles are nice as well.