Layering Maps and 3-D Space
Now you notice this is only a 1-level game, but it's got a couple of maps to it. If you layer maps within the level, you can use the parallax view of the individual layers to give the impression of a 3-D space.
Now I've had the opportunity to look at some of the 3-D engines that are being used by the larger game studios. The cool thing about these engines is that they are not proprietary, you can find them on the Internet. If you wanted to, you could download them.
If you want to create videogames that give the impression of a larger experience than the 1024 by 780 screen, you might want to look into some of these engines.