(I ran into the same problem when building the very first custom tileset for the Newer Super Mario Bros. It seems that at some point during the game's development, the developers found it difficult to fit all of the fancy graphics they wanted into their tilesets. U's visual style involves large, showy graphics. Wii have used extremely similar tileset systems - in each, a level can specify up to four tilesets it wants to use,* and the first of these always contains basic objects such as brick blocks and orange semisolid platforms. All of the games since New Super Mario Bros. U, groups of similar objects (such as grassland objects, desert objects, cave objects, etc) were put into individual 'tileset' files that levels could link to and use. Some history: In New Super Mario Bros.-series games prior to New Super Mario Bros. It is currently being used to make levels for the Newer Super Mario Bros. It handles tileset objects in an entirely new way that allows for never-before-seen flexibility in level design. The Satoru Level Editor is a fork of Reggie NSMBU. Skawo has made a captioned video demonstrating Satoru in action!
The Satoru Level Editor: An improved editor for New Super Mario Bros.
