Retro 1 -
Last One Out is a Rotten Egg Very nice level! Not too hard but at the same time not completely trivial.
Retro 2 - White Chirstmas (sic, title has been corrected)
Oops, unless that was intentional, "Christmas" is spelled wrong 
Biggest gripe here is that unless I severely overcomplicated the solution, the builders are super precise. There's very little leeway in placing them and not getting killed by the ice steamblower trap. I would definitely ease up the precision there!
Retro 3 -
Snowy Wish As mentioned on Discord, I said this level's way too hard for its position because a lot will not realize that a lemming needs to turn around in the basher tunnel. Even more, it must be the climber that needs to turn around. It is way more precise than it needs to be because the pillar is quite thin and so there's very little room for the climber to turn around. You certainly don't want to start a pack off with a harder than usual level, as I believe this puts the pack at great risk of being rage quit.
edit: Armani has made the pillar thinner per my suggestion/feedback. It's a much better level now, and it was a good idea to not make this the opening level of the pack, as the new Retro 1 is definitely easier.
Retro 4 -
Claustrophobia Wow, what a nice level that gives only bashers and miners! My only gripe here is that at the top right where you need to bash to the left it's very unclear that the lemmings can step up where there's a little overhang and they fall just before it. I mistakenly thought it would cause them to turn around. I would make it more visually clear that it won't turn them.
edit: I needed to resolve this one, as I gave Armani the feedback that the nook at the top was a bit unclear whether lemmings would turn around or be able to step up. Fixed my replay there, it plays out the same.
Retro 5 -
So close, yet so far Haha, reminds me of The Tomato Watcher's R2 because of the title

Nice open-ended X-of-everything level here. Bonus points for it being really nicely decorated!

Retro 6 -
Christmas Crossing Nice level where you need to get a lemming on the other side of both OWWs to bash the crowd free. Just me, but I wonder if it can be made clearer that the drops are survivable? Yes, there's always the splat ruler, but I think the precision on the drops can be eased while not harming the level in any way.
edit: Also needed to resolve this, as a change was made to make one of the walls on the side clearer that the fall is safe. Level still plays out the same with where the non-floater needs to go and the floater the other side.
Retro 7 -
How to Escape an Avalanche Pretty nice concept here, where you need to send out multiple climbers and block in the right places so that you can release them all with one well-placed miner. I did have to rewind plenty of times, though, to keep readjusting the blocker positions, especially the leftmost one.
Retro 8 -
Harvest of Screams Great level! I really love the solution here involving two separate digger pits and blocking the left-most one to be released later by the worker who built the bridge to the exit. It took me some time to figure out where to put the pits and when to block, but it's still a nice added challenge.
Retro 9 -
Rendezvous at the Mountain Cavern Nice puzzle, I especially love how multiple workers need to be sent to different areas of the level to do work and prepare the path for the others. It can be difficult to get two of them to fall in opposite directions where they land on top of the sign. The timing is a bit tight near the end as well, but that's probably because I brought that on myself. By this I'm referring to the digger where he almost dug through the builder's staircase to prevent the others from getting to the exit.
Retro 10 -
And There Were Four Again! I was stuck here longer than I needed to because I was a dummy for not realizing that you can safely drop them onto the small steel block at the bottom from the top on the right side!

It's funny how I never noticed this before on this particular level from OL. As a result, I kept coming up a builder short. At the same time, I can still see this one being a difficult one for some, so I guess its position in the rank is fine. Still, nice remake of "And then there were four..." from OL but Christmas themed.
Retro 11 -
Roof of the World Wow, great level!

It was really fun to figure out and solve this level. The only hard part is knowing how to isolate 4 workers to go on and block to turn the builder so that you can send two climbers on ahead, as well as a third one to mine the blocker free who will both become floaters. Also, placing the other blocker where the crowd is compressed so that he can be mined free with a climber later from above. Also, timing the climbers so that the second one turns around in the miner tunnel can be difficult. Overall, this was very satisfying to solve

Retro 12 -
Forbidden Knowledge (Part1) The Part 1 in the title needs a space between "Part" and 1. Somewhat hard level but another great one. I like how one needs to release the pre-placed climber first and then use him to release the pre-placed floater/climber. I think the hard part here is seeing that you need to mine in such a way so that you don't release the crowd because the wall is still too high. I think most would think to mine so that you end up releasing everyone when you can actually afford not to since you can build them out later. Overall, great level to finish off the rank.