Level 8, Mental Process is the one level I can't seem to figure out. I've checked all the steel is genuine in LemEdit, and checked for hidden exits. A lemming can't float through the gap because of the trap. The solution I tried was building from the bottom of the wall to the left, but the lemming hits his head on the metal ceiling before he reaches the floor.
A enjoyed figuring out level 9 thought. That one's fun, and it reminded me of my 'Great Wall of Lemmingland'.
I've looked at level 10, and it looks like a simple task of finding the most economic use of the builders.