Thanks so much for playing my game! what I did was I used one orthographic camera (to render the sprites) and one perspective camera (to render the 3D background). I put the background objects on a new layer. By setting the orthographic camera to depth only and taking the background layer off it's culling mask that camera doesn't render the background and has a transparent background. Then you can take everything off the culling mask except the background on the perspective camera so it only shows the background in perspective mode.