This tutorial will explain how to create a one-way wall; that is, a wall this is only textured and solid on one side, which allows anything through from the other side.

1. Draw out a small brush, (larger than an actor), and change the 2D view to side on (so that the brush will stay at its current size at the next step)
 2. Go to patch>Simple terrain mesh OR simple patch mesh
 3. When the dialogue box appears, choose any density. You only 
 4. You now have a mesh that is transparent on one side, and textured with whatever you choose on the other. Rotate the patch by going to Selection>Rotate>Rotate X/Y/Z.

Properties of the mesh:

 -Textured side: Clips players, items, grenades, rockets, bullets, etc. This side could be made transparent (but not with caulk, this doesn't work properly) with a custom texture, so that it is invisible too.

 -Non-textured side: (shows up as wireframe in the editor) clips nothing, bullets go through, and players can pass through.

Be sure to check the shadows on your meshes and the ones they cast; they won't always be what you expect.

If the mesh is part of a wall, then it can be difficult to make its texturing blend in; you'll probably have to manually rotate the texture (right_alt and right click drag) or move it (left_alt and right click drag).