There are four LEVEL#.### files included with the game. LEVEL0.000, the default, is by far the biggest, but you can play LEVEL0.001 and LEVEL1.000 by running the game with the command line arguments G_MAIN 0 1 and G_MAIN 1 0.

LEVEL9.999 is pretty much just LEVEL0.000 again. According to fc.exe, it differs in only two bytes, 0x226 and 0x228.

G_HELP.TXT is the most useful of the various text files, because it includes instructions for using the level editor, G_EDIT.EXE.