Some comments on the levels:
Trial 1: It already took me a while to save even one single lemming. IMHO this level is harder than any in the Challenge rating. Especially getting the RR right takes some work - I changed the RR exactly nine times during my final solution!
Trial 2: Rather easy and straight-forward.
Trial 3: Still trying to persuade the lemmings to climb a 7-pixel step...

Challenge 1: The most difficult part is to select a left-facing lemming to be the miner preventing the others from vanishing on the left of the entrance. Otherwise a good challenge with an excellently hidden solution, though I would consider the miner-stair and climbing through the ceiling to be very close to glitches.
Challenge 2: Nice and (relatively) easy. A very good challenge level! Only Challenge 1 and Challenge 6 come close, but both their solutions are much harder to execute.
Challenge 3: That wasn't too hard either, though I wonder whether I backrouted it (as I can solve it either with one builder or one floater less). With the given 100 lemmings you have to set the RR to 99 very soon, which doesn't add anything to the level.
Challenge 4: Very similar to my original solution, though I had to add the No-Builer-Turn-Around.
Challenge 5: I already considered this solution, when I encountered the level in the original pack. But I had to play the whole level twice, because during my first attempt I didn't change the RR for the first two lemmings and was at the end ONE second short. AAAAAAARRRRRGGGGGGHHHHH!
Challenge 6: I would consider releasing the blocker this way is extremely near to glitchy behaviour. Otherwise a great way to turn this level into a challenge. I just wonder why one has to save exactly 55%? I also added a replay of my first (non-working) approach, which fails by just 2 pixels. But can you please explain the behaviour of the builder at the very end???

Reverse 1: Introducing Karoshi...
Reverse 2: Is this the intended solution or the backroute?
Reverse 3+4+5: My solutions seem just a bit too easy...
Reverse 6: Not beaten yet, but nevertheless two replays attached. The first fails by a 7-pixel step and the second only has a 63-pixel fall (if I counted correctly). If this was really intended, you have my eternal admiration for an extremely devious mind.

Flight 1: A good way to remind the player of the no-gravity mechanics.
Flight 2: I wonder whether this is a backroute, as I can save 98% (95% being required) with some skills left over?
Flight 3: Again much less skills are needed.
Flight 4: The only thing I dislike about this level is the way one has to start a rather long stair somewhere IN MIDAIR with a leeway of about three pixels in total.
Flight 5: Thanks for the hint "The solution might not be obvious until you actually play the level...".
Flight 6: A nice level, but it is rather obvious what to do. Though I must confess that the addition of the climber and the builder did its job in confusing me and hiding the solution. 


