I am not a map maker, but since map people want to know, here
is a simple description of how to make glass. If it doesn't work,
sorry.

Two Way Glass
-------------
Two way glass can be made by using the clip texture on a brush.
The brush will be invisible, but players will not be able to pass
through.

One Way Glass
------------
1. Create a wall across the viewing area, make
the brush a solid texture (whatever texture you want the players in the
arena to see)

2. Change the face of the brush on the observer's side to the texture 
"skip".

3. You must make it an entity for it to work right, so I used
a func_wall. You have to set spawnflags to 4 so that it gets 
created. 

Observers can see through, players can't, and it clips as well! How
much more could you as for?
