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!

If you make the third lemming out of the hatch a climber, you actually end up only losing a single lemming before the platform is complete. However, I'm still faced with the same problem for the rest of the level and saving the extra lems doesn't seem to help any. I suspect I'm just not aware of this trick you mention. One of the few tricks I know that fall into the 'shouldn't work but does' category is the basher/blocker trick (which LemFan has used elsewhere). This is one of my gripes with his levels actually - there is the odd one that can only be completed by knowing some obscure trick. That's poor level design imo. I'd personally be happy for you to post your replay of this level so I can learn a new trick!

Early on you should be able to lose only 2 lemmings as you platform.

After that, the solution relies on one of those Neolemmix tricks of the trade, where something that arguably should not be possible actually is. I won't be more specific now, as that would give the game away.

Solitude 1 - Prologue Nice level to start off the pack but much harder than I expected despite the 1-of-everything skillset! It's not very obvious at all what skills go where. That jumper assignment is a bit too precise IMO

Solitude 2 - Maverick Somewhat hard level that looks a bit intimidating due to the splitters that makes nice use of the stacker which needs to be placed precisely so that it's a splatform for the Lemming regardless of which direction he enters the portal from. Makes use of the fact the stacker is a few pixels wide. I think it's also due to being shifted over a pixel when popping out of the portal, but I'm not certain.

Solitude 3 - This Erosion Nice tribute to Wicked 3 of ONML. Fairly easy level, with the main catch that you must take care where to bash as you can't necessarily just bash anywhere you want. Bashing in the wrong spot(s) can render the level unsolvable.

Solitude 4 - Building, Fencing Here, I'm referring to building from both sides that Dodochacalo makes use of on one of his levels in Fiat Lem! That apparently completely escaped me here in order to deal with splatting issues in the huge pit at the bottom just to the left of the exit. Very nice level but as mentioned one I feel is too hard for its position. Also the only level to not have a locked exit.

Solitude 5 - Autopilot It should had been clear that the mess of cables around the middle area are all to block backroutes, but nope, I didn't catch on apparently! Solution is quite sneaky with that part on the far right in the starting area, where you need to realize to shimmy and then dig down enough before platforming twice to close the gap so that placed this way it doesn't block the climber from getting back into that area later on in the end once he becomes a neutral and hence cannot be assigned skills anymore. Definitely reminds me of one of Armani's contest levels! Great level here that has quite a well-hidden solution!

Solitude 6 - Bugs in the System Most definitely a backroute

Solitude 7 - The Split Same as the previous level, though not as severe of a backroute it looks like

Solitude 8 - Hanging Gardens Easiest level of the pack IMO, but only because I've seen this kind of solution so many times by now The only hard part here is realizing to bash out to the left at the start so that you can get underneath to do the shimmying.

Solitude 9 - Top of the Ladder Wow, I totally didn't expect to get hung up for a while on this level! Not sure why it took me a while to figure out the multiple platformers to be splatforms that are made by the clone while the digger keeps going. I definitely remember seeing the intended solution to a LemRunner level requiring this. This solution currently seems to require a lot of precision for the splatting to not happen each time, so perhaps consider easing up on it Great level, though!

Solitude 10 - Pillarist Was a big dummy here due to never considering that the laser needs to be fired facing the right rather than the other way, which I was completely fixated on! Needless to say, I essentially had the solution in so many of my attempts but the importance of the direction when assigning skills completely escaped me here! Excellent level with figuring out how to make the path all ready for when he becomes a neutral at the end!

Solitude 11 - Blockhead Another level that I didn't expect to be so difficult! Seems the most important thing here is realizing which buttons need to be gone for first before certain others. I figured my solution is way more overcomplicated than it needs to be, and this was confirmed when I checked Dexter's intended solution after I got the level solved. It's definitely much easier on the timing than mine, where I squeezed out a lot of precise jumping so that he doesn't get delayed by hoisting each time. I do like how two of the buttons require multiple neutrals to be close to one another in order to have one get past the traps guarding them!

Solitude 12 - Undertow Another level that took me a while. The most difficult part is definitely figuring out how to overcome the forcefield in order to collect the swimmer pickup. Another thing that can trip players up is not knowing if you need to dig and then jump twice once low enough after getting the button, or if you can afford to use a builder to turn around instead. Nice level, and another one where I remember a solution to a LemRunner level that requires bunny hopping but to avoid a huge splat pad area instead. This element of the solution took me a while to figure out here!

Solitude 13 - Drowned Rat Another level that is harder than it looks. It's not obvious at all where the builders need to go in order to make a path for the neutral to reach the swimmer skill adder. Also not sure why it took me a while to figure out you can fence to free the blocker and then build to make him turn so that it's impossible for him to get back up later, as well as being too far from the neutral so that he can't get past the trap that's in the water. Nice level!

Solitude 14 - Bell Toll I thought this would take me a while since I tend to struggle a lot with zombie levels, but this honestly wasn't that bad at all. The hardest part would be to make sure you don't block the way to the exit later on with the platformers. It would seem that it's impossible to exit if not swimming due to all of the trigger being in the water.

Solitude 15 - Diving Chamber I consider this the hardest level of the entire pack. I kept running out of skills here and plenty of red herrings the player can fall for here all make for a very hard level. For a very long time, I kept thinking two stackers are needed in the starting area, when in fact only one is needed in order to stop the zombie climbers from going out to the right and infecting the neutral later. In the end, I made my solution far harder than it needed to be, as it requires a very precise jumper assignment to get over all the zombies, though it also saves a stacker. I checked the intended solution after I finally got the level solved and I had a feeling that the laser to make an escape for the Lemming was intended. Similarly, I thought it would be possible to break through both barriers with one laser from the bottom right, I just thought it didn't work at all when I kept seeing it not work I apparently didn't try all possible positions for the laser before and hence it took me a while to finally see that what I thought works does indeed.

As the intended way doesn't require any jumping over any zombies, my suggestion of reducing the zombie amount to just 6 for the one-time mine traps isn't needed at all. Instead, maybe the opposite of increasing the amount of zombies to clearly make my element of jumping over the zombies impossible can do, though of course that's up to you My solution is probably impressive enough to be acceptable though

Solitude 16 - Disjoint Disunion Nothing hard about this level other than realizing that the key here seems to be to delay the cloning assignment for as long as possible so that the one who goes to the exit chamber doesn't go past the exit before all buttons are activated to unlock it, as he won't be able to come back later due to passing through the skill remover and hence will drown.

Solitude 17 - Babel Not a very hard level either, though here I suggest making the neutralizer near the exit more visible, since it currently looks like it could be mistaken for a trap that would kill Lemmings instead. Of course, I brought this on myself of wasting so much time when I could had simply activated CPM much earlier, but I honestly like the challenge of sending a Lemming over to the right spot to see what happens After a while, I turned on CPM just to make sure what I was seeing in the level are indeed what I think they are, in particular the splat pads were as I thought they are. I happened to mouse over the object in the very narrow gap near the exit to see that it's a neutralizer. Even then, I still didn't make the connection that you're supposed to ignore the Lemming that becomes a neutral and disarmer once you reach the button near them at the top. Funny thing was that shortly after I exited out of NL I then realized the importance of the neutralizer near the exit But yea, I would definitely make it more visible Granted, it might make it more obvious to players that you can ignore the objects at the very top, but honestly I don't think it makes it that clear in any way

Solitude 18 - No Looking Back Not a really hard level but still a well-hidden solution makes it fairly challenging. Took me a while to realize you're supposed to platform at a spot so that when fencing to the top from the left that he'll be able to platform twice in order to reach the platform and turn around, since the way it currently is the platform with the skill remover is lower than the platform on the left and hence the platform bridges won't touch it. Very nice! I also like the added touch of the skill remover taking away the floater skill of the zombies so they can't swarm the exit later. Great level!

Solitude 19 - Silver Mines Ok, this level currently demands way too much precision in the starting area with the builder bridges. I thought this was the solution, but I was quick to dismiss it when I saw it wouldn't work. Thus, that made me think that making a splatform on the bottom left was the solution for a while The solution really reminds me of an old version of tan x's Jungle World 10 from LemRunner which I know Icho criticized. So, my suggestion of reducing the precision required applies for this level as well I honestly don't see how the level can be backrouted with the precision reduced or eliminated and hence why the demanding precision confuses me greatly. Keep in mind that requiring a lot of precision in solutions isn't necessarily the best way to increase the difficulty of levels, especially on an engine where the difficulty should come from the puzzle, not the execution

Solitude 20 - Daybreak One of the best levels of the pack IMO! The main trick here is mining to get under the triggers of both teleporter and buttons in order to make a turnaround point but cancelling with a shimmier at the right spot under the triggers in order to still reach and activate them. Nice!

Solitude 21 - New Leaves Contender for hardest level of the pack along with Solitude 15. It's essentially Solitude 2 but with terrain changes and a different skillset to indicate the passage of time since the original level it's based on. Not sure why it took me a while to realize that one can use a jumper instead of a walker to turn around on the top left even though I realized the jumper to turn around somewhat quickly for the top right side The splitters on the level can be intimidating here as well! Great level but another one in which I would suggest reducing the precision needed for the jumper/shimmier transition on both sides!

Solitude 22 - Epilogue Just like the previous level, this one is based on the layout of Solitude 1 but once again with terrain changes and a different skillset, as well as the permanent skills are provided in the form of pickups. Another level I say is a contender for hardest level of the pack along with Solitude 15 and 21. As it turns out, I made a few incorrect assumptions which definitely got me into a lot of trouble here and hence prevented me from getting the level solved. For example, I'm not sure why I thought you're supposed to fence through the wall next to the bottom left button when you can just climb back up from the other side just like the way he climbs over to get to the button Similarly, I'm not sure why I never considered just building over each gap the Lemming comes to at the start instead of making him a slider to get down safely Once I started looking at the level differently, then the solution naturally came slowly piece by piece as I saw new stuff I hadn't tried before and eventually culminating into a solve! This level does a great job of teaching you how important it is on when to assign the permanent skills so that you don't lose them to the skill removers before you need them to be taken away. Another level which I consider one of the best of the pack and an excellent one to finish the pack on!