I just saw this on Flickr. This is great! I do think some foliage would be good to help break up the area of blank green baseplates. Also, the green bird gets a little lost - is there some way to offset him? Hmm, what if you put the catapult and the pig fortress on slightly raised brown sections, like here:
That might both break up the 'ground' area some and also give some color background for the green bird.