I just hope no one will ask us, "Why did you put those two names in the same category?"
They are the exact same world, just different names for it.
Summary
So, first of all. Radiant Garden is the world where most of the characters lived. Why did I say lived? Because
9 years before Sora's adventure, 1 year after Birth By Sleep, Xehanort and friends sent away the leader of the world, Ansem
the Wise, opening up a chance for Maleficent to strike. This world was not ordinarily a Heartless world, or at least one particularly
strong in darkness, no, this world was very peaceful until a year of an open world heart caused the darkness to corrode it,
be released by Maleficent, who then took over the world as her way of obtaining the supreme darkness. Funny how she was after
the wrong thing the whole time.
So, after Maleficent controlled the world, and all the residents fled because of the Heartless population
threatening to steal their hearts, she began seeking the rest of the Princesses of Heart. It's known that Maleficent started
her plan during the course of Birth By Sleep, after Master Xehanort told her about what a connection the princesses have.
It's odd how this is the only world that needed some sort of "password" to get to the heart. Like, you needed 7 Princesses
of Heart in order to release the powerful darkness inside. No one really understands why this is. It's believed that Ansem
the Wise put the code on so his apprentices wouldn't be able to use the powerful darkness within, although nothing's confirmed.
Even before all this, Xehanort joined up as Ansem the Wise's apprentice, and him and the other 5 began
research into the mysteries of the heart. They got some test subjects who lost their hearts during the experiments and created
the Heartless, the first time these creatures have entered the Realm of Light. It's believed that Lea and Isa, curious as
they were, snuck into he castle to see what was going on, and ultimately lost their hearts because of the Heartless. Of course,
they might have been the test subjects as well. Anyways, Isa obviously got much worse of it before losing his heart,
and it changed him into the cold Nobody Saix.
After the apprentice's lost their hearts, ASod (Ansem, Seeker of Darkness) continued the reasearch and left
Radiant Garden shortly after.
It's said that upon the release of the world's heart, brough upon the new name, Hollow Bastion.
Many other characters resided in this world, too. Kairi, and her family, Leon, Yuffie, Cid, Aerith and Merlin,
even Donald's uncle, Scrooge McDuck all resided there.
