I think "What makes a level challenging?" on the pure puzzle side comes down to 2 things:
1) Entropy
2) Trickery
1) "In statistical mechanics, entropy (usual symbol S) is related to the number of microscopic configurations Ω that a thermodynamic system can have when in a state as specified by some macroscopic variables."
Now my definition of "Entropy" for Lemming levels
:
Entropy can be described as the pure ammount of possibilities (chaos) you have in a level. Of course a 20 of every skill level is usually not difficult, but I am rather talking about levels with a barely enough skillset that have a high ammount of
possible (not actual) ways to solve them.
For example a level with limited skills that has great hints where you HAVE to put the skills has not a high Entropy. Like for example you have a water gap (no swimmers) and only 1 builder --> the builder HAS to be here --> lower Entropy.
If every skill of the level has multiple places it can be used in a manner that gets you really closer to your goal, that's the type of level that is really difficult. (If you have elimminated all backroutes and only the intended way is left of course
)
So: Higher Entropy ---> Higher Difficulty (example would maybe be the 2nd last level of Reunion "Lemtris Version X" or the last levels of NepsterLems with a high entropy and an easy 1-of-everything level with clear usage places for every skill with a low entropy)
2) Of course difficult tricks increse the difficulty of the level, but there is another thing to be aware of here: If a person knows the trick a lot of the difficulty can get lost. Therefore concealing the trick should also be a goal. Sidenote: A level where the place for the trick is clear and maybe even be hinted at would be a good introduction level for it.
Let's take the last level of the first rank of Reunion for example:
In it's first iteration the starting area did not have it's metal plates --> the blocker or a digging skill was the obvious choice for containment.
What I wanted the player to use was a builder block: 3 builders after each other to make a 1-way wall and create a delay for the 1st lemming. This was considered to be too wasteful for most of the players --> good concealment of an easy trick.
I added the metal plates to lower the entropy (to hint at rule 1) and make the trick more clear --> blocker + digging tools were no longer an option and the level was now easier.
As you can see here is that point 1) Entropy can also be a great tool to hide your tricks
. The other way would be in the terrain shape. "The Glacier Station" from the 3rd rank of Reunion would be a good example for hiding sth in the terrain without unfair methods.
Be careful with concealment: Don't actively hide things from the player. Hide it in plain sight! It's always visible, but not clear to the player. A bit like a "Where's Waldo" picture, just now you are searching for a solution or a place to use a trick in a helpful manner.
So the trick complexity (this also includes combination of tricks and maybe even multitasking) and trick concealment is a key to more difficulty.
I hope this helps and I got the main points to create difficulty.
I would like to maybe hear other peoples definitions.
Maybe I missed a point.