Level 6 took me the longest time to figure out — I somehow convinced myself that something was a failstate and took an eternity to question that assumption while looking for complexities!
Thanks for playing!
Yes, level 6 is the hardest to solve, which is partly because it looks similar to level 4 and you may be tempted to solve it in the same way, but also because conceptually it requires a new idea. But conversely, or because of that, I think it's also one of the most satisfying to solve.