Thanks so much for this art, Surt. I am using it in my upcoming game Pixelsphere http://pixelsphere.org. I have a remix coming soon so people can adapt the version of the tileset I am using. I am also sharing selected assets from my game to give back to the community to thank you.

Your comment about Copyleft is still a bit cryptic, I am sharing my additions to the art under copyleft as well, but obviously not the code and other copyrighted assets to my game. (so I would choose the CC-BY 3.0 license)