A couple of points about "Slow and Steady":
It would be quite a good level to do a hard version of, because you can actually do it with _much_ fewer builders than you're given. Make lemmings two, three and four erect a "bridge wall" under the trapdoor.....
And there aren't many levels like that on the original game! Don't forget that when I made these levels I didn't know there were such things as programs that let you make your own levels (actually, back then there probably weren't). I know it's an obvious idea.... but I had to do it, just because it's the sort of level I'd have a lot of fun playing!