Whilst scouting about for some simple floor plans to experiment with in Google Earth I came across a site about Affleck Castle in Monikie, Angus. It has an interesting description of the castle complete with a set of floor plans. A little image editing was required and then it was a simple matter to stack the plans.
The effect is achieved by setting the white areas of the images to 50% transparency and leaving the black areas opaque. I tried several alternatives to this scheme - another that looks pretty good is to make the white areas completely transparent. This remains readable as GE leaves a fine white outline.