The other day, to have a look at how it's really done, I decompiled Karith's .bsp into a .map and took a look. The page I got the relevant command from said there might be a few errors in decompiling it, and I noticed things like unaligned textures.
The thing I noticed most, however, was that all the fantastic detail on the walls was made up of structural brushes, not detail brushes. Is this an error created by the decompiling process, or is Karith doing something weird? I always thought it was best to make brushes detail if they didn't touch the void - the way Karith's .map looked, there would have been thousands upon thousands of vis portals.