No, you didn't spoil it. From the tools given it didn't look like you'd be able to save everyone and with only one builder which needs to be used to get up over the steel I think some of them will fall into the shredder trap, or just be wandering around aimlessly by the time 2 minutes is up. And if it's possible without the builder then my guess at having a gradual slope by getting the Lemmings up top to dig in sequence from left to right must be along the right track.
Edit - Yeah, I've got the solution. Haven't been able to execute it yet but I know it works. Now for Final Impediment.. when I'm building from the first steel box to the one to the left of it, no matter how far left I start my bridge, it always goes one step too high and the rest of the Lemmings will just walk right off the edge. Is this where the Bashers come in to slightly extend the first bridge laterally so that it doesn't go too high?
Edit 2 - Never mind all of that. I got 80 Lemmings home in I am A.T and just now I beat Final Impediment. I got over (no pun intended) the problem of Lemmings scaling the wall by digging while building my bridge, that was enough to lower it to make the gap too high. Thanks to ccexplore, guest, the guys in this thread and everyone else who helped me out on a few levels when I got stuck. ^____^