Suggestion additional instructions from players after release:
Make sure that you can hear the music from the menu screen before beginning. Sounds are key here, for feedback and understanding the gameplay elements. (Future balancing for various systems might be needed)
If you are have used up all the light crystals in a level and can not reach the portal, there is a chance that your light was used in the wrong combination. It's very possible that you'll have to restart the level (esc + restart) in order to complete it. This is especially true in the 'secret level' which add the energy drain mechanic.