I finished it, but noticed that there was at least one part where it looked like I was supposed to go back and take an alternate branch route. I couldn't finish the alternate branch at the time and went on to finish the game, but now can't go back to try to explore the branch again from the last save point. Just wondering if that's intentional? Aside from that question, it's a very good puzzle game and one where I haven't seen that take on sokoban before.
Edit: Looks like I'm first to comment on the secret levels :3
For anyone who needs a walkthrough: https://3p0ch.newgrounds.com/news/post/1358512