Seriously. This is definitely above and beyond the usual. No crazy patterns, no violent blinkingness, no creepy face to spring out at you.
Go to the site, and check out the castle by putting your mouse cursor over the image. Notice that it's entirely black and white. Move your mouse pointer off to one side of the image. Now, just like it says, stare at the dot in the center for about 30 seconds, and don't move your head or look away. Relax your eyes even. Then without looking away, move the mouse pointer back into the image.
When you blink and it goes *pffT!* back to black and white is pretty trippy.
Really, really cool optical illusion