Yep, once you clear 18 it's smooth sailing, to a disappointing extent. 7, 12, 14, 17, and 18 were the ones I found the most difficult, and I never got 12 or 18 without help (I got within two moves of 18, then went the wrong way from there, so close and yet so far). Backtracking definitely helps, you just have to figure out how to do it. It's also sad that my 500 MHz iBook could be brought to its knees by a flash game, but level 20 did it.