To make bits of your room invisible - assign different materials to them (in UVmapper for instance) - then you can set them 100% transparent when not required. For things like doors and windows, a quick cheat is to make the door or whatever be a separate prop parented to the room. Then you can just use rotate or translate to open and shut it. Clunky, but it works. If you want to go the whole hog, you'll need to get into hierarchy files and joint parameters... shivers ...but something like this would be an easy way to get started.