/vr/ - Retro Games

Mapping question. After the player spends time puzzling his way out of a hellkeep, he finally opens the door to the outside, only to reveal that it is right in front of this "aztec-esque" temple stairs where the exit is on top. I know you make sky by setting that F_SKY ceiling texture I already have in the picture, but how do I make the walls seem "transparent" with vast landscape outside. (I know the stairs/walls are currently not properly textured, they're just my map defaults)