I think castle oblivion didn't just pop up there, it has to be a reason in which they are there. My theory is that Land of Departure is actually castle oblivion!?!?!? yeah I know it sounds crazy but think of how we saw in the trailer that L.o.D was swallowed by darkness, but who said that it vanished? Maybe thats why those rooms are there with Ven and Aqua in it, because they were trying to save their world. Just a thought...