How to find a switch/linedef that opens THIS door?

One of the advantages of updating from an old editor, in your case Doombuilder2 (DB2), is that you can enjoy new features like eventlines. Use either GZDoom Builder (GZDB also old now) or better still GZDoom Builder - Bugfix (GZDBBF).

Go in sector mode, highlight the door's sector, and you should get an arrow going from the linedef that activates it.

Special cases when that doesn't work:

• Door tagged 666 is actually opened by shooting all Commander Keens or killing some boss monsters
• Door is actually opened by a script (ACS or FraggleScript). This normally shouldn't be a concern for a vanilla-, limit-removing-, Boom-, or MBF-compatible map since they don't have scripting, but you can check the map for scripts anyway.
• Door cannot be opened because of map bugs. It happens sometimes.

In this case: it's opened by line #28736, which is activated by a voodoo doll out of the way. The voodoo doll, in turn, is released by activating the switch on line #11790, which is all the way north and slightly to the west of the door that intrigues you. Near a plasma rifle and some Hell knights.

Map is Man on the Moon, not "Mood". https://www.doomworld.com/idgames/levels/doom2/Ports/m-o/motm

23 minutes ago, Gez said:

Go in sector mode, highlight the door's sector, and you should get an arrow going from the linedef that activates it.

@Dimon12321 is using DB2, that arrow is a GZDB feature.

But you can still find it out in DB2:

1. Go to sectors mode and highlight the sector. Note the sector tag (130 in this case)
2. Go Mode -> Find & Replace Mode (or press F3). Select Linedef Tags as search type and enter the sector tag from the previous step (130 in this case)
3. Click Find. You'll get a list of Linedefs that tag the sector.

22 minutes ago, boris said:

Dimon12321 is using DB2, that arrow is a GZDB feature.

It's also a feature in SLADE. I didn't realize DB2 lacked it, I haven't used it in so long that it just seemed like a core feature of all modern editors.

1 hour ago, Gez said:

Map is Man on the Moon, not "Mood"

My error. That's right!

I have moved to GZDBBF and now I see more.

32 minutes ago, Kappes Buur said:

One of the advantages of updating from an old editor, in your case Doombuilder2 (DB2), is that you can enjoy new features like eventlines. Use either GZDDoom Builder (GZDB) or better still GZDoom Builder - Bugfix (GZDBBF).

 

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Before I choose the best answer, could you please show what does this linedef (2523) exacly do? I know it will raise the walls around this switch (2428), but how to make the builder light them up?

Found it. So the voodoo doll crosses those linedefs in front of it and they raise the floor and teleport monsters.

Thank you guys for your answers!