Allow an athlete down, then build to seal the gap. Place a blocker to isolate one lemming on the left side of the starting area. Have that lemming build over the water into the one-way ground while the athlete builds to avoid the trap at the bottom. When the athlete finishes climbing the steel wall, have it build left into the mound then right onto the platform such that there is no gap. Build into the one-way ground, again leaving no gap, then bash through it. Blow up the blocker to allow the rest of the lemmings to make their way to the exit.