>>714840498
Try thinking of the game structure like a Super Mario World zone. Each chapter is a self-contained level and once we beat the game and unlock them (via the Pure Crystal, finding the shelter codes, working out why Gaster says PREPARATIONS ARE COMPLETE when you copy one file to all three slots, learning how to break the game from Noelle, or being able to swap control to Susie etc etc) they'll have secret exits patched into them. Finding a secret exit will take us to a secret level - which like the other chapters would be its own self contained game.