I was wondering how lights are applied on to walls so that you can see the wall behind the lights through the alpha. (I'm not too good at explaining, but like those square lights on the turret thingies on top of the bloodgulch bases.) By the looks of it it is just a plane slightly offset from the BSP, but I can't see how that keeps the map to the sealed world rules.

It's a decal, open up the bloodgulch.scenario and look for the decals section(in the edit types window), once you find it you can find its location and put it in your map. Hope this helped.

The lights are not decals. They are transparent shaders that are part of the bsp.
The light is a plane that is render only so it fits the sealed world rules.

^

Its a piece of geometry with the light bitmap uv'd onto it. The material name has the "!" indicator telling the engine when compiling that the piece of geometry with that material does not have collision. Thus, it will now compile, and the shader for the light will take care of the rest.

also remember as a geometry piece it needs to be attached to the frame the same way as you just attach the bsp to frame.

ty