Close-by observers see one castle and far-away observers see an other castle. There's also an old version with faces, which I found even more impressive: http:

Close-by observers see one castle and far-away observers see an other castle. There's also an old version with faces, which I found even more impressive: http://farm1.static.flickr.com/171/435058989_9d83960682_o.jpg http://farm1.static.flickr.com/171/435058989_9d83960682_o.jp... Take a few steps back and the faces swap! I have seen optical effects used on billboards but not this exact one, yet. Perhaps it's hard to achieve a good effect at billboard size.