they all fucking mess up by allowing me to see the skybox when i open the door and once i pass through it, the room i just left appears to be the skybox..
You are not doing anything wrong.
This should explain what a area portal does if you do not already know.
In my own words, an area portal is a controllable "door" in the vis leaf parts of your map.
But let me take a step back and fill in some gaps for questions that may arise.
In your Hammer Editor, with your map already loaded and compiled once, go to Tools > Load Portal File > Click yes on pop-up.
What you will notice in your 3D view are a bunch of blue lines all over the fucking place. The volumes (3D areas) created by these blue lines are your vis leafs. When you compile your map, hammer checks the visibility by checking, from each individual leaf, what other leafs that leaf can see. If a leaf is in no visible sight of another leaf, it is not rendered from the leaf the client is standing in. That way, a client would not need to render the whole map when it cannot see all of it.
Of course, when you compile your map with Hammer, it automatically chops up your level into vis leafs.
If you have a sphere somewhere in your map and you compile, it will chop a single room into over 500 vis leafs.
The same thing goes with cylinders and any kind of diagonals / details / small brushes.
Simply select your small detail brushes and press cntrl + T to tie them to an entity called "func_detail".
This excludes them from the vis compile process and can drastically reduce compile time and gain performance.
Only non-entity brushes will be considered when hammer is compiling for vis.Now that you have cleaned up your map with func_details, lets say you want to further optimize your map.
You can do so with hint nodes and area portals, but those are advanced and shouldn't be used unless it is desperately needed.
You can use google or the link I gave you to understand what area portals do.
A good example of an area portal is on the map cs_assault.
The upper back door in the alley way uses a area portal in the doorway.
When the door is opened, the area portal opens and vice versa.
This not only gains performance for clients but it also prevents wallhacking as anything in vis leafs that are not visible are not rendered.
I hope this helps you and anyone who read this.
If you need any further explanation, whether you or anyone else, you can PM me through forum.
