Okay, I've been searching for a certain level for quite some time.

I am going insane. I have not found it in Final Doom, Dom 2, nor Ultimate Doom, but I am certain I played one a long time ago. it started with heavy gun zombies coming out of a wall, and there was a cyberdemon outside a window later on. There was another part with chain gun guys across a courtyard firing at me. I am 95% sure it was not in a mod though. If anyone can recognize my description, could you please tell me the level name?

Never played completely Final Doom for PC, but i'm sure the level you're talking about is on Final Doom PSX, level in a bright courtyard, with a few shotgunners (or chaingunners, can't remember) out of walls opening when you try to access a key.
On the start of the level, there's a chaingunner starting to shoot point blank in your back on ultraviolence.
After that, there's a sort of "arena" with a cyberdemon and chaingunners all around.
Maybe the final level of 1 episdoe of Final Doom PSX, can't remember if it's masters levels, tnt or plutonia.

I played it on PC though.

There's a comparison here.

That description doesn't sound like Vesperas, Heck or Onslaught though.

I don't understand. This is quite an enigma. maybe it was a custom wad. I don't remember getting one back then though.

I found the level with the heavy gunners. It is the hanger level in TNT. Now I have to find the other one.