You know those walls when they are fully transparent on on side and passable, and on the other side a textured wall that you couldn't pass through, generally saying, one-way walls? I forgot what option it is in and I tried search >.<
Also once when I did put in a one-way wall it's placed in the centre of a square (8-pixeled), so it doesn't seem to fit well with the walls and could be easily seen as a trap, is there any way it could line up with one of the edges of a square?
I once made a cube and made it hollow and then ctrl-z-ed it and when I looked outside it looked normal but when I went inside it was totally transparent, though I still don't know if it is passable like the ATCS glitch build?
Edit: Another thing, how do you make a two-way wall with a texture in it that is passable like Karith?