I know this is a TF2Maps WIP thread.
But tell me what you think of this concept;
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Be careful with 3d skyboxes though, displacements (and normal geometry?) do cast shadows on the actual playable area despite being in an isolated box somewhere else. The magic of the sky_camera does that.
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Isn't that mountain(s) in the skybox from MaxofS2D?
A BUILDING
It also fills your physics data really quickly--you can see this in the compile. I don't know why you'd needlessly bloat that.