The first few levels were nice intros and fairly easy. It ramped up pretty nicely, too.
Level 7 seems incomplete, because you can just ignore the lemmings that are trapped and still win the level easily. I feel like you should have to save them.
Level 9 could be made a bit more interesting if you had to save all of the lemmings. The pillars don't touch the ceiling though, and I'm not sure why the release rate is locked.
The decoration is a little samey. Many of the levels were built mostly out of rectangular blocks, and there were some pieces I didn't see at all. If you're going to restrict yourself to just one tileset, you should try to make the levels look a bit more varied, I think.

Hearing your chiptune cover for level 9 was refreshing; you should use your chiptunes for all the levels!
