The key to Cascade 100% is that a (pixel-precise) well-placed digger can dig out small parts of a step at the long staircase at the start, such that it's no longer possible for lemmings to walk up and past the altered step (they'll turn around instead), but without also completely digging through creating a gap, so the lemmings won't start falling off either.
Now that you have a way to hold off the crowd from falling to their doom, the rest logically follows.
