A little thing I noticed.

red beam? it's blue. it's really, really, really blue.

the picture from the big bang video is flipped, it's looking at it from the opposite side as nath, which lends more credence to it being from the victoria island side. if you look, the two side pillar lengths are left short, right tall on the nath picture and left tall, right short on the video picture. it's the exact same castle without a doubt, just from the opposite PoV.

Looks really red to me. Anyone confirm please?

Also, the side pillars are left tall right short in both the pictures, without considering the top part, which looks the same iin both the pictures, so I don't think it's flipped.

It looks like a powerful energy is originating from the castle in the trailer, even the clouds above it are spread apart by the force from the beam.

If it's not something just for show in the trailer then I'm sure it'll house some sort of powerful boss or something in it (Possibly Black Mage like mentioned already) The castle appears a second time in the trailer where it seems like they're heading toward it and then monsters suddenly appear as if blocking the path to the castle.

This is the best shot I could get of that part:

 Spoiler

Didn't notice that one. If they show it TWICE, then, I'm pretty sure there must be something there.