to DWT, Nanookz!
I went to The Magic in Pixels.com - Home to see a really good picture of this year's Castle Dream Lights, and sure enough right over the upper windows there is what looks like a big letter A.
I don't believe it stands for anything - if you look at a photo of that part of the Castle in daylight, you can see there is a piece of architecture there (forgive me, I have no idea what it would be called) that is a pointed triangle with some details inside. It looks like they tried to follow that architecture with the Dream Lights, and ended up with the "A" effect.
It's in my photos from last year, too, but I never noticed it until you mentioned it.