The game is split into 9 chapters. Each chapter has a theme and associated obstacles.
Each chapter, except chapter 6, has a couple of strawberries. Strawberries are often off the main path and act as optional challenges. Collecting strawberries is entirely optional and does not affect progress in the game.
Hidden in each chapter is a cassette tape and a crystal heart. Collecting the cassette tape unlocks the B side version of the chapter, while hearts are required to make meaningful progress in certain chapters.
B-Sides act as more challenging versions of their base chapter. B-Sides do not have collectable items, except for a crystal heart at the end.
Chapter 8 acts as a post game challenge and requires crystal hearts to progress more than a few screens. The A-side only requires four hearts, while the B-side requires all other hearts available.
Completing the B-Side of chapter 8 unlocks C-Sides for all chapters. The C-Side of each chapter is a shorter, but trickier challenge. Again, C-Sides have no collectable items, except for a crystal heart at the end.
Chapter 9 is the post-post-game challenge and was added to the game a while after it was released. While some progress can be made in it when it is first unlocked, all crystal hearts in the game must be collected to access the majority of it.