the additional rocks within the ground bring interesting variety, but they're -- once again -- very rectangular. They should not only serve the purpose of distracting the eye, but also to break the "cubism" that is naturally induced by tiling. this post (http://www.wayofthepixel.net/pixelation/index.php?topic=4925.msg78092#msg78092) in the building of Lectro game illustrates what i mean, so i take the liberty to cross-post souly's art
(http://i34.tinypic.com/dmtjq1.gif)
you'll note that the below image only feature "vertical", "horizontal" and "corners tile, and that it gives a fairly rectangular look to the pice. On top-right corner, Souly took more liberty with intermediate and corner-filling tiles that gives the cave a much more interesting look (imvho).