Let's maybe start from the visuals. I really love those pixelated graphics that appear in this game, with the combination of red-black color palette, where the only differently colored object are the main player, and the treasures you can pickup. The graphics are pretty unique, seem as a whole, and probably are the best part about this game!
Sound effects are okay, but I do have a problem with the music. Now, I do appreciate that you made your own soundtrack for this game, and not just one or two tracks, however, the music isn't particularly good. It's repetitive, after a while gets very annoying, and just seems like something you'd make in Bosca Ceoil on your first time using it.
The gameplay itself, I would consider very fun, if not for one, major issue, but let's not get ahead of ourselves. Controls feel responsive and smooth, and the addition of air jump is a good one! The levels themselves, are also not bad. At first they might be a bit hard, but after a few tries, they usually are very easy to beat, and completing them without taking any damage, does feel very satisfying. The only minor issue with the gameplay are physics, which sometimes might not work correctly, leading to such results as sticking to the floor, and being pushed back when standing next to turrets and other sprites.
And now, let's talk about this major issue with the gameplay. That issue being, restarting from the first level, after every death. I know, you wanted to create a difficult game, but that just doesn't work! Needing to spent a few minutes after a death that sometimes might be accidental, just to repeat 10 levels you've already played tens of times before, isn't challenging, but rather frustrating and dull! And the solution was very simple: just add checkpoints! Maybe not for every level, but rather for every 5 levels, and then the game would have been much better!
That one issue, completely ruined the experience for me. Which is a real downer, because besides that (and maybe the music) the game is good! But for now, I consider this game slightly above average, it definitely needs some more development and polishing.