Oh, I see now! The most unobvious thing for me here was that I should return to the VERY beginning, back to the first location. Collected all 5 fireworks, so it works:) BTW The ability to go through passage between locations exists by default, no need to take powerup for it beforehand.
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Great, thanks for giving it another go! You're right, it looks like I forgot to actually add a check in the code that you have the doorway powerup, lol. The game is supposed to be a metroidvania, but since my game is so linear up to that point I totally understand that needing to go way back isn't obvious. I was running out of time before the deadline to put more work into the level design, but for this genre especially I wish I had dedicated more time to it.