In this map (it was map 9 in the previous version, at least), I'm not sure how to advance.
https://i.imgur.com/RoeE9qZ.jpg
I have the yellow key but I couldn't find the trigger where to use it. By noclipping, I know I have to open up this bars first. But the switch is on the interior side. I could pass by crouching but that would be cheating obviously...
from the other side I cannot pass through here to reach it.
https://i.imgur.com/UPfgcUJ.png
In general, the mapset has some flow problems. Like the final switch that frees some Cyberdemons in the previous map in the final combat arena, it wasn't that particularly visible. Or in some levels there is a switch that open up a door for x seconds in some other room and it isn't clear what's happening (nor there is a clue to guide the player). Or textures that are doors and it isn't clear so you can miss it, it may seem obvious to you as you created it, but even if it has that side metal bars, it's a big level and it may not be obvious to the player. I would add a pair of flame posts flanking it.
https://i.imgur.com/6m2y7da.jpg
Or perhaps the most puzzling design choice, you use color codes switches in a totally different way that most creators do: instead of indicating that switch can only be used once you get a red or yello or blue key, it's a switch that can be used without keys and it means it will activate something that will help you in getting that color key.