so…
we should make as much detail brushes as possible, but not these which touch the outer walls.
what if i make a huge box around my level which ofc will be structural, can i then make everything inside of that box detail? :poke:
i have no idea 
also, does it do any “damage” if i make enties (sp…) detail?
You can’t make entities detail, it just doesn’t do anything so no harm done.
First of all, what do you mean with “but not these which touch the outer walls”…? That is wrong! Detail brushes can touch the outer walls. There is no problem if they do.
Your idea with a huge box around your map can work, but only if you have a VERY small map.
Let me explain what detail vs structural actually does, although you can also find alot of info if you search it in this forum 
First of all you need to know a little bit about how compiling works, and how the game renders/draws your map.
If the game would draw the whole map at once, at an average of 70 frames per second, that would need a LOT of computer power to do!
So the solution is that not the whole map is drawn at once, but the areas of which the game is sure you can’t see them right now, are not drawn.
So basically, you only ever see a small part of the entire map, the rest is just not drawn.
During compiling, the game does some calculations on what areas can be drawn when you are on position x and y etc…
It does this by dividing the map into blocks or rooms.
Suppose the map is divided into room A B and C.
If you can draw a straight line from room A into B, then that means that if you are in room A, you can see room B.
And also, if you are in room A, the game has to draw both room A and B, but not room C.
Here is where detail and structural brushes come in.
Basically, structural brushes define the blocks, while detail brushes don’t.
If you make the walls of a room detail, that means that the game thinks you can always see through them, meaning it has to draw alot more of the map, and it runs slower.
If you make them structural, the game doesn’t have to draw what’s on the outside (provided there are no windows ofcourse).
On the other hand, if you make a chair out of structural brushes, that would create probably about a dozen new blocks which have no use whatsoever, which again means the map runs slower.
If you make a chair detail, these blocks are not created and the chair is ignored during the calculation.
A basic rule is to make small things such as chairs and stuff detail, and make walls floors ceilings etc structural.
So to conclude… Read up on structural vs detail, and possibly try to find some information about “HINT” brushes, which are closely related.
And to answer your question, making everything detail except the outer walls will thus force the game to draw the complete map at any time, even if you can see only one room of it… A bad idea!
that clears some stuff up.
well, in my case im building a TJ map which has many closed rooms whose can be reached by teleporters.
so i can make everything in them detail as long as i leave the outer walls structural.
thats pretty much all i wanted to know
thanks for that explanation
Yes, trickjump maps are probably the only exception were you can make everything except for the walls detail (as long as you use teleporters).