I know you can't add planes as collision because Halo don't like that, But I've seen some BSP maps using planes as terrain and of course, the BSP has collision (No portals, is terrain).

I want to know how can you use a plane in a BSP, I've been thinkin to flip the faces in the box but that's just the same, Halo will render it anyways.

Thanks

Halo needs a skybox and must be closed off. It probably wasn't a plane, it was probably just a flipped box with the skybox on all sides except the ground.