I got stuck on Level 13 yesterday, came back again today to see if I could solve it but I couldn't. Great game though, I just wish I were able to finish it =)
Check the other comments. I posted a walkthrough for level 13 (it is also on my youtube channel). There are two tricky bits. 1) there is an opening on the ceiling in the darkness. To see where it is there is a light sphere to push on the left. Looking where it falls makes clear to see where to go up. 2) Going forward there are small openings where to push light spheres that will light the path in the water to reach the end of the level.