So you want to make walls invisible? If yes then there are few ways how to do this. The best one (used in diablo3 too) is to define meshes which are too large. In 3d software, break those big meshes into 2 pieces, one piece will remain, second will be hidden. Then from camera to your character, use sphere collision trace. if this sphere trace collide with mesh you want to hide, then just hide second bigger piece, or you can make it smoothly disappear with some settings in material