It is possible to do semi transparent textures. House did it a long time ago, but hes not around here anymore, and I never got around to fiddling with it. As far as I know it should work just fine and allow you to see whats behind certain walls. But yeah, if you change the opacity of textures on buildings you will see the empty space inside. Just use noclip to poke your face through a wall, and thats what you will see.