as the name says, this is of course the debug file! this area isn't able to be accessed through the normal game, but does lead into other in-game areas (the black woods, elemenatry, gems n' geodes, and possibly more that i haven't found). there are some interesting little details that weren't used in the full game, for example when swampy can interact with an npc, the npc will have a little talk bubble above their head!
