OK
I made my own map and figured it out. Indeed it's rather easy to implement.
Half Life is in the 3rd dimension, the 4th is next door. Everything seems harder to understand in this eerie area between these dimensions :).
Thank you
Half-Life - Making a Wall That Blocks Player But Lets Monsters Through?
Is it possible to make a wall that blocks the player but allows monsters to pass through?
I have been experimenting with "clip" texture but it seems to block the monster too - I though that was the case if the "monster clip" flag was ticked?Thanks