In an Dengeki and Marukatsu Megadrive magazines interview, game's graphic designer Toshio Yamamoto stated that his team didn't use a planning document for the game.
Contributed by ProtoSnake
In the Japanese version, by pressing A,B,C,A,B,C,A,B,C in rapid succession on the Options screen, an easy mode will be unlocked. The game will be significantly easier, but the first level of the game will change to a light and cheery aesthetic instead of the dark and moody one of other difficulties.
Contributed by KidDivinegon