This is some really nice work. To get such a strong sense of theme and place within a whitebox minimalist art set is quite an achievement. Also, thanks for confirming for me that my speakers do work by actually including sound FX, but I'd advise prioritising FX in future since the most necessary things (flames, crumbling platforms, the player) didn't have them!
Overall, there's just not much to criticise here ... it's a smooth journey with a few big difficulty spikes (sorry, I couldn't get past the rising lava in the 4th level!) but there's a great consistency and a neat rhythm to the experience, with good use of enclosed spaces and a steady beat of action/respite.
One major point, though, is the constant use throughout the levels of branching paths. It feels wrong to me - you're giving the player the option of missing out on some fiendish challenges. The optional paths don't feel challenging to find so ultimately why offer them? Be proud of your work and guide the player. You should be confidently stating to the player "here is my work, enjoy it!" not cautiously saying "here is a thing, maybe, you could try it, if you like". I didn't find any of the "optional" paths that I thought couldn't have been part of a system of progress gating that would force the player to collect everything and experience everything, and I think the game would have been better for it!
Good work, but be proud of your level design and make use of it.