Okay, here's my next challenge (and likely the last good one for now, since I currently don't have any new ideas past this one).
Design a level with the following criteria:
1) 4 entrances
2) initial RR 99
3) requires every lemming saved [note: this means solutions using nuke glitch doesn't count]
4) For each skill type, you are not given more than 20 of that type of skill.
(ie. you can't have 21 diggers, but you're welcome to give 3 of each skill type even though the total of 24 is greater than 20. You can certainly give less than 20, including zero ie. no exploders for example.)
5) No one-way wall objects allowed.
6) If you remove any one or more of the entrances, the resulting level becomes unsolvable, even with the following additional changes to the level:
a) initial RR lowered to 1
b) total number of lemmings increased to 80
c) you are only required to save 1 lemming
d) for each skill type where you were originally given at least 1 of that skill, increase the count of that skill type to 99
e) infinite time limit (basically, the level cannot become unsolvable merely due to a potential solution running out of time to complete)
For rule #6, note that you need to ensure unsolvability for all combinations of entrances to remove. There are 4 ways to remove one entrance, 6 ways to remove two entrances, and 4 ways to remove three. All 14 resulting levels [each with the additional changes outlined a) thru e)] all have to be unsolvable. In summary, the removal of entrances is so detrimental to the level, that it's not possible for you to scrap together any solution even with all the added advantages outlined a) thru e).
[edit: no changes to rules, but added emphasis on rule 6c based on various attempts observed]
Note that Lemmix does not handle ordering of 3-entrance levels correctly. The correct ordering should be A, B, C, B, A, B, C, B, ..., but Lemmix under most configruations do A, B, C, A, A, B, C, A instead. For purpose of this challenge, we will use the ABCB ordering for 3-entrance levels, NOT Lemmix's wrong ordering. You can achieve this in Lemmix by making a copy of the second entrance. For example, if the original level's entrances are A, B, C and D in that order, and you want to examine the level with entrance B removed, you can get the correct ordering of A, C, D, C by selecting entrance C, do "copy", and do "paste" to add a duplicate entrance C object.
Hope the rules are clear, and good luck! Post here if there are any remaining questions on the rules of this challenge.