what exactly does it do. people tell me to use it, but what does it do?

I love func_wall, but i use func_wall_toggle more. I like to use it instead of doors to block off certain areas. If you play anyone of my maps, sf_toxic_final, sf_drainage, etc you'l see the walls blocking off certain area. And when fight tiome starts they just dissapear without a trace.

Heres how to use them.

1. Make a wall and turn it into a func_wall toggle(group them together if its like the same thing(ammo wall, build wall, devider))

2. If the wall is for build only times(blocking the other team from getting to your map, hiding the ammo) then set it's flag to initially_invisible. (in game it toggles all items first, so it's really invisible, he he)

3. In the logic_gametimer just make entries for all walls for onBuild & onFight, Just tell the walls to toggle, no open or close stuff here.

I like walls a lot more then doors, they have no moving parts.

func_wall is extremely useful in making unbreakable stationary glass and stationary solid matter. I think func_detail takes the place of func_wall as stationary solid matter, but anyway, always tie extremely complex brushwork to an entity, such as func_wall, as opposed to being just a world brush. (Func_walls do not "seal" your level though, they are not world brushes)
A good use for func_wall is making small walls in a room. If the wall is large and blocks your vision, however, leave it as a world brush to be a vis blocker to increase performance.

So:
Make a world brush a func_wall if it is small (And doesn't block much geometry)
Keep a world brush as a world brush if it is large enough to block a significant amount of geometry.

The trick is to keep vis leafs reasonable.

To be honest, I really don't see the necessity for using func_wall, especially on fairly plain maps people make for SourceForts.

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i find func_door best cuz it simply is pwnage

Yeah, my next map will have no func_detail's, the whole thing will be a series of doors! How cool would that be!

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I made a map that was only a 1024*1024 box with a wall in the middle, and a glass door, I put various env_cubemaps near the glass and it made it look a little nicer, but the compile times were shit.
Was that just the env_cubemaps or should i have used func_wall for some of the inside brushes?

the cube maps as knifa said dont add to compile time, and dont really do anything unless you have a < dx9 vid card and you do "buildcubemaps" when you are actually playing the map.

you might want to check what kinds of textures you are using. and what the lightmap scale is. i heard this from someone shaved 10 minutes off the compile time. click the upper left corner of the 3d view where it says "camera" and set it to light map grid. and open the texture application tool. itty bitty boxes on things means it calculates each of those boxes for lighting. set the "lightmap scale" on things like the skybox and stuff to high numbers like 512 or 1024 in extreme cases.

btw, func_wall is listed in hammer as "legacy support" which... im pretty sure means it just there for help with porting maps.
hammer also says to use "func_brush" instead.

I am a HL1 mapper as well...I do HL1 things in HL2.

so if you change the lightmap scale then it calculates light for a large area not all the tiny ones?

Is there a way to increase the scale of a texture, like for making "mouse" levels where everything is huge.