What about the wall on the right? How did he do that? It's as if it's an animated image, just just changing positions of an image.
i think stretching and squeezing the parts near and far the mouse position does/would do the "depth" effect. too bad he didnt do it for the y-axis as well.
Edit: And images: http://www.romancortes.com/ficheros/meninas_sprites.png http://www.romancortes.com/ficheros/meninas_bg1.jpg
Edit 2: http://www.romancortes.com/ficheros/meninas_bg2.jpg