Here's how you make a 1 way window/mirror:
First set up where you want the window.
You're going to need two seperate brushes/panes of glass for this to
work, so make sure you have the 2 brushes flush against each other in
the place where you want the window. Make sure both brushes are
textured with a glass texture, like GLASSBLUE1. Now, you want the
two brushes thin enough so that they look like they're just 1
brush. Here's the view from above:
Now, on the side which you want to be able to see through select the
visible brush and tie it to func_wall and set the Render Mode to
Additive. Change the FX Amount to something like 150, so that the
glass is transparent.
Now go to the other side of the window to the part you don't want to be
able to see through. Click on the brush facing you and tie it to
the func_wall entity. Except this time you're going to set the
Render Mode to Solid (no light). You also want to set the FX
Amount to the max of 255.
If you were to jump in game now from the transparent side you
would clearly see that there was another pane of glass (because it
wouldn't be see through). The trick here is to apply the {BLUE
texture to the side of the non-transparent brush facing the transparent
window. So when you look through the transparent side you'll see
right through the piece of glass (since the {BLUE texture makes that
face invisible), but if you walk around the other piece of glass is
still there and non-transparent.
Here is the view of the solid brush textured with {BLUE on that single
face. I've hidden the transparent brush so that you can see, but
make sure both brushes are visible when you compile:
Hoped that helped! If something was unclear, ask.
