Hey,
The graphics are really nice, and the story seems interesting. As you said, there is no puzzles or challenges, and my tip would be, spend less time on the pixel art graphics, and just use some circles and squares, get all the mechanics, challenges and gameplay working, and at the end add the graphics. I'd much rather play a game with no graphics but great mechanics than play a game with beautiful graphics and no actual gameplay. It would really benefit from audio, because the nice graphics set a mood for the game, and I think audio would take it a step further. My last point would be, I don't see how it really fits with the underground theme, maybe I'm missing something.
Anyways, well made, nice controls and good player speed, lovely graphics. Good luck in the game jam :)