Doombuilder 2 help

I need help learning how to lower walls when crossing over a linedef

OK, would you please provide more information? Like, which format are you using, for which game are you making map, which methods you tried.

I need help learning how to lower walls when crossing over a linedef

That depends greatly on the mapping format you are using.

For original DOOM select the triggering linedef, to edit it, and open the Floor category



Scan through the list for anything W1 (walkover once) and find a linedef special which lowers the floor.
Select the one which comes closest to want you want to happen. For example:



Note that the floor sector and the linedef special must reference the same sector tag.

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For G/ZDoom, both DiHF and UDMF, you have many more options



For example: Select a linedef to lower the floor in one go or lower the floor incrementally
or lower the floor instantly or at a slower rate, etc.

As before, note that the floor sector and the linedef special must reference the same sector tag.

Or you could elect to use a script (ACS or ZSCRIPT) to have one or more actions take place.

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Several things to consider:

• Read Doom Builder: An Illustrated Guide to learn how to use the editor and the mapping features. Download it for offline reading.
• Upgrade from Doombuilder2 to either GZDoom Builder Bugfix (DirectX9, same as DB2) or Ultimate Doom Builder(OpenGL 3.3 or higher), depending on your system's graphic card
• The ZDOOM WIKI is your friend