The reason why it is difficult for me to explain is lack of my English vocabulary.
SıмΞоп in US SmileBASIC community has tried to make mandelbrot set drawer and his coloring algorithm is much better than mine. I recommend you to check his play diary.
Each sprite has its own unique management number, and it'll be available after you call SPSET with its management number. If you call sprite related command (e.g. SPOFS) with management number before SPSET, Illegal function call will occur. But you say syntax error occured. I guess you misused SPOFS command.
This is mandelbrot set, a set of complex numbers that has a highly convoluted fractal boundary when plotted.
This screenshot seems very complicate, however the recursion to draw it is actually simple.
I love such strategic tank game! It reminds me of battle city on NES.
In my opinion, Japanese SmileBASIC users have no permission to attach photos on US SmileBASIC community.
It's very simple method. It lines Z axis with 64 sprites on which vertical section of terrain are drawn. If you break execution of this game with SELECT button and you see GRP4 layer with smile tool, you can see sprite fragments of background image.
Weak point of this method is so slow execution. It costs approx 2 sec to draw an image with legacy 3DS.