It occurs to me that if you actually could use multiple tilesets in one level, people might create tileset files containing nothing more than a single platform type, single cave system, single textured background, single background layer, stuff like that, so downloaders wouldn't have to waste time on lots of unused tiles in the tileset from unused layer stuff. So if you wanted to use the temple in Nature's Ruins, you'd just use the "Nature's Ruins - TEMPLE" tileset and not force people to download the grass and the mud and the background and stuff. Interesting...