Everything inside Ultimecia's castle obviously represented the darkness version of Ultimecia. I mean, Ultimecia lacks her backstory in VIII, so why not add something new to make her place valuable? Of course, if her castle looked exactly the same as some regular building from the present era (Squall and Co.), then that would be even worse!

But the star doesn't exist inside Ultimecia's castle as it actually exists outside. There's proof: the Moon outside of her castle looks exactly the same as the Moon from the present era.