To chime in on this, I just released several videos showing how Solus levels were built, including one in-depth video with some of the actual content from Solus.
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://www.hourences/
I would not advise to build stuff in brushes and then converting the brushes to meshes. That is not really proper and asking for issues long term. The way we normally approach building levels is using modular meshes, with a couple of unique meshes here and there for big things + possibly a small amount of BSP for simple and unique structures. This is equal to how UE3 and UE2 handle it, so you can also look up any information about UE2 and UE3 to get a feel for how we approach building stuff.
In this level I built 6 years ago you can see the process how I went through. I use BSP brushes only for the biggest simplest stuff. The rest I do with modular pre-made meshes. View it as lego. You build using lego pieces, and adjust yourself to the pieces you have available.
://www.hourences/tutorials-the-spire/
I also have this video up that is a timelapse of how an entire level is built from scratch, so you see the entire way I approach things. I started off by building very simple pieces in , the type of stuff you’d almost do as brush, and then worked on using those meshes.
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