Fake walls

Hi im wondering how to make a fake wall. A wall that looks like a normal wall on the outside but that u can walk trough and is invisible from the inside.

The two sided linedef that connects two sectors (or how ever many sectors you have in the area) needs to have a "middle" texture on one side.

A few threads below this one has already answered the same question. Click here. Might be of use to you. ;)