Bevel and shadow are the only way I can think of to convey the impression of height on the wall. It's not *realistic* to have an internal wall casting a shadow (i.e. there's certainly not a light source *behind* the wall to cast the shadow) but it fools the eye a little. Don't make it too long though...maybe just use your wall layer and create a dropshadow layer style so you can mess with it as you develop the interior. GIMP has layer effects/styles? Sorry, don't know the app.

For the texture: I assume this is underground? If so I'd just have the wall texture to the thickness of the wall and use a dirt/rock texture filling the rest of the map. Make it nice and dark so it doesn't distract from the main content.
M