Levels 4 and 6 are so good! It took me more than 2 hours I think.. I understood 4 completely before solving it, spanning several days. The 6 I solved on conceptual level and the Idea behind it was beautiful, and I feel that it is just top puzzle design (ever created, or smth), but. But most of the time particularities of mechanics stand in the way of high lvl concepts and force you to bruteforce most of the lvls instead. You get an idea, but the fact about the movement of elements and their relative position doesn't allow to test it. Still pondering this thought..