In the areas where you have used Plateau, the edges look spiky and unnatural, so hes telling you to use the smooth tool on them to make them look more natural. As for my suggestions, I would suggest that you make a bit more height variation in all of them, especially in the 3rd one. Also try adding some fog, and add some doodads in the background, especially in the 3rd one, because it looks like theres nothing behind the wall.