Spanish Castle illusion

I love optical illusions, particularly the ones that make use of ‘persistence of vision’. (You know, that little ‘quirk’ of our vision that gives us the illusion of continuous movement when we’re actually seeing a sequence of static pictures, as in a flip-book or a movie.)

Do go and try this one:

Spanish Castle illusion

(BTW, although you’re ’supposed’ to stare at the dot for 30 seconds, I’ve found that five or ten seconds is usually long enough for the illusion to work.)