By the way.. because I'm still discovering new keys and things... does func_door has key "angles" like in buttons? 

Yes, but like buttons, only in the horizontal plane, and from what I've seen it can get confusing if you use non-orthogonal directions.

Doors make great lifts and extending walk ways, you just have to learn how to control them by setting the "lip" value. 

So it is better than func_plat in many cases?

I tried using now skip, I think I asked about whether it makes same effect when using invisible textures.. and it makes the same weird effect, it kind of don't want to draw some faces:
https://drive.google.com/file/d/0BwxYkKdSD855eDdtQU1JdEpPSE0/view?usp=sharing 

From Quake Wiki...

"Skip is a nonstandard special texture, used by modern QBSP compilers, but does not have any function in the stock Quake compilers as released by ID Software. Skip is a texture much like clip in that it is invisible, but skip's use is to remove an entire face from ever rendering, and a brush does not have to be fully textured in skip. The removed face remains solid, and will block player and monster movement, as well as block weapon fire. In general, this is of little use on solid geometery, as Quake performs backside culling, but can be used on brush entities or "hacks" like windows to hide unwanted faces. A "*waterskip" texture exists as well, for use with liquid brushes."

So you use it only on those faces of a brush that you want to remove, not on a whole brush. 

I don't want to remove anything, I only want to let monsters move surfaces. 

Thanks for who mentioned about func_wall + alpha.. it worked! 

Looking at your screenshot I think you have a grid, and that you want monsters to be able to cross the grid?

If so, then you cannot cover the whole grid with one skip brush. You need to fill each of the spaces in the grid with a brush of exactly the same size as the gap, and with the same texture as the grid, then retexture just the top and bottom faces of that new brush with the skip texture.

After compiling, the faces with the skip texture will be solid but invisible, meaning that you can see what is below the grid, and all the other faces of that brush will remain, thereby making the grid look complete. Monsters will then be able to cross the grid.

I used that technique in FMB-BDG2 where I had a grid with some holes big enough for the player to fall through if he was not careful, and skip textures everywhere else so that the monster could chase the player across the grid and maybe panic the player into falling through one of the bigger holes (those bigger holes did not have skip textures in them). 

The problem in the screenshot is that the skip brush is world geometry. This is what I explained above. The skip brush needs to be a brush entity, ideally func_wall (NOT func_illusionary). Alpha brush is a bad choice, because when playing the level with a port that doesn't support alpha, that brush will be fully visible. 

When world brushes touch, regardless of texture, the touching faces are removed. If top and bottom face are skip-invisible, then you can see through the sides where the brushes touch. So in order for this to work, there must be no touching (similar to the "DP" issue), for example if the skip brush is shaped like an upside down pyramid or at least has slanted sides to make sure only the floor surface matches up with the adjacent floor. However, this is problematic as well, since it may lead to the parts below or behind the skip brush to be visblocked and therefore create a visual anomaly, too. 

"Alpha brush is a bad choice, because when playing the level with a port that doesn't support alpha, that brush will be fully visible."

Didn't some just said Skip is not supported as well in some ports.. or am I blind. It seems there is couple ways to solve this problem. That I don't know what is the right way to do it anymore.

You can recommend using specific engines for your maps.. if it comes to that mentality "it should be working on every possible quake engine" you're just making your life unnecessary hard?

I just tried func_wall + alpha and it gave the best result when using Quakespasm. So if that is the wrong way to do it, then shall be it - if it works on this specific engine.. 

Do AD's fence textures not work in darkplaces? Or have I borked something somewhere? That screenshot is 100% less pink in QS.

Actually, should I care about DP at all? I only installed it tonight to see how my map would look (basically the same save for the broken plants) and experience the horror that is excessive particles and blood decals. Does anyone care if my map's broken in it? 

Sorry about that; my memory is shot!

Negke is right. Ignore my comment about exact size and retexturing, that was the actual problem I had, not the solution. The solution was a standard brush 1 unit away from grid brushes (still needing one skip brush for each gap in the grid)

I think this shows what I am meaning:-

http://imgur.com/a/PzDvZ

You can see the skip brushes in place, and the larger holes that the player could fall through, but monsters would not.

This screenshot is from the original map that was compiled and worked as intended. 

Actually, should I care about DP at all?

No. 

Like this? : https://drive.google.com/file/d/0BwxYkKdSD855amlBS1FCSWVlaHc/view?usp=sharing

Is the offsetting by 1 pixel only way to make it not hide faces from those iron bars? 

Let me get this straight. Skip is a compiler thing, so if the faces are removed does the engine port used to play the map make any difference?

Alpha does require engine support, if not supported by the engine port used to play the map the model will appear as normal and not transparent? 

Yes, that looks right. And yes, the 1 unit gap stops the grid's faces from being removed whilst allowing monsters to cross the grid.

In my map, Ogres, Death Knights and Shamblers could chase the player across the grid. 

should I care about DP at all?

It depends whether you intend to allow players to play your map without forcing them to put their engine of choice away or restrict it to some kind of elitist club. Your call but IMHO mappers shouldn't make their work dependent of one engine. 

You turn it into a func_wall so you don't need to have 1-unit gaps, because brush *entities* and world geometry don't affect each other in this regard. Yes, it's also possible to have skip-textured world brushes which then must not touch other brushes, but it's not recommended in most situations. There'll be lighting issues, too.
And you only need a single big func_wall that covers the whole space between all the bars in that area - since it's invisible to the player, there's no visible z-fighting.

Rick: Correct.
Mugwump: Correct. 

Dp got fence texture support a few weeks ago, it's only on the "development auto builds" (haven't tried myself but should work): https://icculus.org/twilight/darkplaces/download.html 

Including dosquake to my knowledge 

"And you only need a single big func_wall that covers the whole space between all the bars in that area - since it's invisible to the player, there's no visible z-fighting. "

but still this happened? Did I do it poorly?
https://drive.google.com/file/d/0BwxYkKdSD855eDdtQU1JdEpPSE0/view?usp=sharing

Though I don't remember was it func_wall or not.. have to see again.. 

Yeah, it seems to work. Thanks negke. 

So now it is come to this point I need to ask again about these keys (AD specific): _shadow, _shadowself, _dirt.

Can some one of those make this brush work as func_illusionary.. like it doesn't block lights? 

I had no idea that was what you could use func-wall for. If I ever try to map again I will have to find a use for my new-found knowledge :)