I love the lighting and depth in the sky, as well as the attention you've put into the background, despite the sky bound castle (temple?) being the main piece. Its a wonderful design and the amount of detail in it is just amazing. The people on the hill are a nice touch to bring a bit of perspective in, though my favourite part is the myriad of birds (Here's hoping they are birds). I can't even describe it, but they just add something more to the piece.
Well I have to ask what is this castle!! My first impression was that this castle is the home of the gods, and the are returning to earth to let the humans see that they're insignificant. Or something like that xD Wooooooonderful work.