I've finally added more layers to the parallax backgrounds system, so the environment looks more natural now. I've optimized the images so the file size (game executable and the installer package) is not visibly larger. The boss level now looks like an arena with an audience.
Next? Creating more enemies for the world#2 (only have gelatin cubes for now), creating tile sets and then designing the levels and putting it all together so there's two worlds (settings) with more than 15-20 levels.