Very good art! It's nice to know you made everything by yourself. When you go back to the surface, it feels a little weird because you don't reset the speed to zero when the character hits a ceiling. By holding space under a platform you can even eventually go through the platforms and go up with an absurd amount of speed.
Also, when I pressed escape while trying to pause, it brought me back to the main menu and I had to do everything again.
Other than that it's a pretty good game. Well done!