Congrats on your first game! Good start, great atmosphere and visuals, and fitting music.
You definitely got it right by creating the player mechanics before building the level; it's best to dial in player movement and actions, then carefully build the world around the limitations of those mechanics. I'm not sure how intentional this was, but double-jumping rapidly launches you really high, which would be a neat feature to use in your level design (i.e. having timing affect the characteristics of the jump). Keep building and experimenting with this; looking forward to playing what you create!
Also thanks for hanging out in chat during the jam!