@joe45: I should look at the scores before asking questions like that... weird, I have tried out every reasonable position of the cannon, and a few unreasonable ones as well, and it never worked for me so far. :( And the problem seems to be that the cannon is sitting a tiny bit too high, so the turtle gets distracted by the top wall while jumping, that's why I asked whether the cannon maybe was placed wrong.
Are you sure the cannon is not placed wrong? Is it possible to get a jump from that cannon into the hole in the wall on the left side? I have tried countless times now and it just doesn't seem to be possible.
Extremely hard. I just can't get past that first cannon that comes down after you got the star. I think I have to shoot the turtle through the opening on the left side, but no matter how carefully I aim, I always hit the black spikes. Very strange... because the way the beam goes, it shouldn't hit anything.