1. a ton of testing. usually its testing to find out exploits (unintended sollutions) and to see how hard it is. i go with. if i am able to beat it.. its to easy. (or for most others. its already impossible and i make it even more impossible).
2. it actually happens quiet often that it seems it was to easy or to hard. but thats has mostly to do with me being surprised how many people know about the trick i implemented.
stop spamming trash levels. thats my advice. id easily can say. 90% of levels are made by people who put in now thought in it and its either a poor grind fest (you needing to expand for 50 turns until you see the AI just to totally destroying it then) or its poor luck.
an example to the luck one. there is a differents for me when someone makes a scenario with 39 AI's and random starts and 1 big bonus that basicly wins the game and to levels like lepanto that while its a ton of luck it atleast thought through it. maybe hard to explain when you dont play his levels.
4. a puzzle challenge. usually its either an "whaaat?" moment (good example can be this https://www.warzone.com/SinglePlayer/Level?ID=1161882
) you start out the level and you see that you can do seemingly nothing to beat the AI. its.. like the name says.. a stalemate.
or i try to trick the player in a way that they will do something where they are 1billion % sure about they have to do it but in fact its losing the scenario already.
i also make generally very hard levels. wich is pretty much the reason i only play from a view selected players levels. those being DanWL / Lepanto (if he would make a none 99.999% luck level) and Muli.
everyone with their own distinct style
Lepanto: invented the "pure luck" setting in contrast to the "pure skill" one.
Muli: basicly makes their level luck free and still very difficult.
Dan: plays around with settings
Me: puzzle's and tricks
thats can be seen very good when you sort it in difficulty as as of right now luck is the only way to make an extremly hard levels wich is why you will find first Lepanto a lot.. then Me.. then Muli... and then DanWL
maybe one day i find a way to make an incredible impossible level wich requires close to no skill.