The 'Prepare' rank was quite easy and very enjoyable, the levels are beautifully decorated and often reminded me of levels from the original lemmings game. As an added challenge, I tried to save 100% on each of these levels, which was also very fun.

My favourites from this rank are probably "Tree Trunk Labyrinth", "A Sacred Vault", "Down in the Root Cellar", "A Little Flower Walk", and "Valley of the Mushrooms". The latter three especially were quite fun since they were quite tricky to save every Lemming. I also really like the designs of "Construction Function", "The Steel City" and "Caught in Liminal Space".



I found the 'Nuisance' rank to be much harder, some of the later levels really got me thinking. My favourite is "The Chosen One". I think it's really great level to end out on due to its high difficulty. Solving this one felt really rewarding, since it forces you to think critically and piece together the solution based on the odd skill selection and save requirement. "Mesmerizing", "Hoppin' On the Groove Line", "In the Rear with the Gear", "Glow in the Dark Pool" "Deep Chasm of Coinage" and "Terrarium" were also really fun, although I wasn't able to get the talisman for the latter.

By far the hardest for me was "Mirror World", it took me hours to solve, and I'm fairly certain my solution is a backroute, but I'm at a loss for ideas on how to proceed otherwise. "Down the Rabbit Hole" was also a really tricky level that I think I backrouted.

Also, just FYI, in the level "When the Whistles Blow" the steam on the far right side of the level doesn't kill the lemmings - it needs to be 1 pixel lower so they don't walk under it.

Shantytown - I'm surprised that you managed to find yet another solution for this level - I really didn't expect this level to have so many unique solutions due to its very limited skillset. It was quite fun to watch!

Vive La Revolution - Damn, another solution in which the zombies don't get guillotined... Well done - your solution of using the laserer to cause the zombies to drown was really creative!

Long Shot - Both of your solutions differ from what I had intended, especially the first one. Great job for finding these, I thought for certain that the talisman solution for this one was completely airtight!

Pumping Station - This one was made by my brother. Your solution differs heavily from the intended one, but it's quite ingenious! Very well done!

Pushing Up Daisies -  Both your solutions here are seriously impressive, let me try to explain. The solutions that you did here are very similar to what the original talisman solution was going to be - relying on precise timing to get the lemmings to fall off the side of the platforms to hit the switch. I changed it in the final version, adding shimmiers so the player wouldn't be reliant on timing. The fact that one of your solutions managed to do it without using shimmiers is mindblowing, well done!

Dragon's Nest - Another one by my brother. This one was very fun to watch due to your unique solution.

Ring of Fire - Your solution differs quite heavily from what I intended, but all of these levels are designed to be somewhat open ended. Well done for finding this solution!

SIMPLE:
Fields of Gold - Really creative solution using a stacker staircase to go over the arrows at the start, well done!

House of Fun - This one's always a pleasure to watch people solve due to it being very varied with a limited skillset for the talisman.

Alabashter - Another very nice solution, nice work using a miner to make the staircase at the last obstacle.

Methane Meadows - People often get confused about the many restrictions for the talisman for this level. I should've made the talisman more explicitly clear - it's classic skills only, so no skills that were added in NeoLemmix.

Stepping Stones - Completing this one with only using 3 jumpers is quite impressive. The intended solution is to use the jumpers to jump from platform to platform, but I don't feel the need to enforce it too much, since this pack and especially this rank is supposed to be open ended. Well done!

Dodgy Plumbing - The basher staircase, as well as the miner/digger staircase in the second half of the level to obtain the talisman was fun to see!



SAFE:
Let's Get Down to Business - Another solution which was fun to watch. Very nice idea using the compression method to keep all the Lemmings from spreading out too much and losing time.

Diamond in the Rough - This talisman was originally gold, until I realised that it's much easier if you let the last Lemming collect the cloners, much like you did here.

Enchanted - WOW! This isn't actually the intended solution for the talisman, but it's seriously impressive. Well done!

Doors and Gates - Another level I love watching people solve, especially because of its deceptively tricky talisman. Great work!

Cursed Treasure - The neutralizer is here to enforce the 'one Lemming goes ahead while the rest stay behind' trick - it's very difficult to complete if they all go at the same time. The skill adder is there to make it harder to save every Lemming for the talisman.



TRICKY:
Shantytown - This level also differs somewhat from the intended solution, which I didn't think was possible due to the limited skillset. Well done for finding it!

Which Way, Mr. Scarecrow? - I'm surprised that so many people found this talisman difficult, it was absolutely not intended. There's a trick to win consistently that involves turning a Lemming around at the exact moment it touches a scarecrow. Still, nice solution and sorry for the tricky talisman!

Beware The Serpent! - As Kaywhyn pointed out in the comments, you did a similar solution to him which differs from intended. Still, it's a nice solution and you did disarm the snake. (This level mainly exists to show off the snake disarming animation which I'm proud of and think is funny )

Vive La Revolution - Boo! You didn't guillotine the zombies! (JK, any solution is valid, well done for finding this one! )

Fear Knot - Yet another level which is always fun to watch. If I recall, every person who has solved this has found a different solution



TREACHEROUS:
Long Shot - The talisman for this level is really nasty, and as far as I'm aware there's no other solutions. Great work for finding it!

Pumping Station - The level is by my brother, so I can't really comment on the solution, but this one's always really fun to watch. It's an absolute nightmare marathon to achieve the talisman, very well done!

Sacred Cloisters - Another talisman that I underestimated the difficulty of. Well done!

Bamboo Grove - This one is also by my brother. The reason it's surprisingly easy for its position is due to issues I had with level spacing - (I didn't want two consecutive levels to have the same style, resulting in odd pacing)

Pushing Up Daisies - Nice one! This one differs slightly from the intended solution for the talisman, but it's still impressive to watch.

Subjugation - Very impressive solution - you saved way more than was required!

Heard it through the grapevine - Another level by my bro. He's very proud of this one, and for good reason - it's an outstanding level like you mentioned. The talisman solution is correct, well done for finding it!

Ring of Fire - I'm very proud of this one. Glad you liked it, and very nice solution, even though it differs from what I intended.

The extra builder is just a spare, as the terrain can be a bit awkward. It isn't relevant to the solution.

QuoteI'm definitely a fan of portals!

Me too! 

Prologue - Nice solve! This actually isn't the intended solution, however I do not consider it to be a backroute as all skills are used.

Maverick - Intended solution, great job!

This Erosion - Nice work. Fun fact - this level is actually a holdover from Lemrunner - originally there was going to be a jungle level with the same name and layout as a reference to ONML, but was cut as I felt it didn't fit the vibe at the time.

Building Fencing - Great work! This one was especially rewarding to watch you solve. This was one of the last levels I made for the pack and was honestly intended as a filler level. The original version used miners instead of fencers, and was much much easier. I felt as though I had already made so many miner levels and very few fencing levels, so it was completely remade and became significantly harder. I'm quite surprised with how well it turned out and it's since become one of the levels I'm most proud of. Also, this level does have a locked exit, but now that I think about it, the button is completely superfluous. It's right next to the skill asigner that you need to pass through anyway, so removing the button wouldn't really change the level in any way.

Autopilot - This solution is actually not intended. However I do not consider it a backroute for two main reasons. 1 - it still is in the 'spirit' of the intended solution (building a bridge and digging down to avoid the trap). 2 - it is more convoluted than the intended solution, so I don't really feel the need to modify the terrain. Great work for finding this!

Bugs in the System - Backrouted. This will need fixing. Good work!

The Split - Another backroute. Fortunately, this seems like a relatively simple fix. Good job once again!

Hanging Gardens - Nice solution! I love the builder/shimmier trick because it's so satisfying to watch. I can never resist making a level which utilises it.

Top of the Ladder - Another solution which was extremely rewarding to watch you solve! The idea for this one actually came from my brother - when I first told him I was making a new pack, he sent me a very simple level concept which utilised a slightly different version of the digger/platformer trick which I played around with and modified into this level.

Pillarist - Yet another one that I enjoyed watching. This was the first level I made for the pack and another one I'm very proud of. Well done

Blockhead - Great work! This one's always tricky due to the tight timing.

Undertow - Another great job on this one. My brother also commented that this level was similar to the LemRunner level when I first showed it to him, and that did not occur to me when I was making it. 
(Another fun fact - the four big stalactites in this level point to where Lock must start building to make the jumps easier)
Also, whoops! My name has a typo in the author of this level.

Drowned Rat - A level with an open-ended solution. I always enjoy watching what people come up with for this one, since like you said, it's not obvious at all where the builders need to go.

Bell Toll - Another level with an open-ended solution. Nice one.

Diving Chamber - Differs heavily from the intended solution, but is not a backroute. (I have different criteria for each level for what I consider to be a backroute. For this one, the intended solution is that the player must trap the neutral lemming inside the 'airlock', then free him later, which is what you do here. Good job!

Disjoint Disunion - Another nice solution.

Babel - This is a huge backroute, but fortunately it seems easy to fix. I agree that the neutralizer towards the end isn't clear enough, I'll replace it with a big one in a better location. The whole point of that neutraliser is to make it so you can't bypass the trap, but you managed to do that anyway! Great work, it seems I massively underestimated you!

No Looking Back - Another one which was very fun to watch you figure out, great job! This was also one of the very last levels made for the pack, as I was struggling to come up with decent solution that utilised the disarmer.

Silver Mines - Nice solution. Sorry about the precision! The distances in this one were precisely placed in order to fix an old backroute, however I'm not sure if that backroute still exists due to having the starting platform slighltly raised. I'm reluctant to change the terrain too much as this might ruin existing replays, but I'll try and add some context clues to make it more obvious/easier to build/jump, like a slightly elevated platform at the start.

Daybreak - Thank you! Another one that I'm hugely proud of and one that was fun to watch!

New Leaves - This, along with epilogue were both intended to be super tricky. The precision needed for the jumper/shimmier was originally more lenient, but it made the solution too obvious, so I changed it to make it harder. I'll have a look to see if I can make it slightly easier, but make no promises. Great solution!

Epilogue - Backrouted! Damn, I thought for sure this one was completely airtight. Fortunately, it seems like an easy fix. Well done for finding it!

a skill adder, it will change any lemming that passes through it into that skill.

Autopilot - This is a similar backroute to what IchoTolot discovered. I thought for sure I fixed that! 
Diving Chamber - Very creative solution! I hadn't considered using the gaps in the right side of the level to turn the Lemming around. This has been changed in the latest patch.
Epilogue - WOW! This solution is extremely creative! Unfortunately, I do consider it to be a backroute.

All of the new replays are the intended solution!

00 Prologue - Backrouted... I may need to rework this one
01 Maverick - Differs from main solution, but uses all skills, so I consider it valid
02 This Erosion - This one doesn't really have a set solution
03 Building, Fencing - Intended solution
04 Autopilot - Backrouted, but the backroute is very impressive! Fortunately this one seems relatively easy to fix.
05 Bugs in the system - Intended solution
06 The Split - Intended solution
07 Hanging Gardens - Intended solution
08 Top of the Ladder - Intended solution
09 Pillarist - Intended solution
10 Blockhead - Has no set solution, any route is valid
11 Undertow - Backrouted! Fortunately, this one also seems like an easy fix.
12 Drowned Rat - Has no set solution, any route is valid
13 Bell Toll - Has no set solution, any route is valid
14 Diving Chamber - Major backroute - the level will need a complete rework
15 Disjoint Disunion - Differs from main solution, but uses all skills, so I consider it valid
16 Babel - Very impressive solution! Differs slightly from intended solution, but not enough for me to consider it a backroute.
17 No Turning Back - Intended solution
18 Silver Mines - Intended solution
19 Daybreak - Intended solution
20 New Leaves - Intended solution
21 Epilogue - Intended solution