Hollow Bastion is one of the few worlds in theKingdom Heartsseries that is original and not a Disney property (next to the Destiny Islands, Twilight Town, and Traverse Town). True to its home series, though, its lore is complicated. It was not always known as Hollow Bastion, and its original incarnation is instead known as The Radiant Garden.
The games it appears in includeKingdom Hearts,Kingdom Hearts: Chain of Memories,Kingdom Hearts 2, andKingdom Hearts: Coded. As The Radiant Garden, it has also appeared inKingdom Hearts: Melody of Memory.For those wondering why itis in so many of the titles, it is because the location has always played an important role in the story.

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As Kingdom Hearts' Radiant Garden
Before ever becoming Hollow Bastion, the world was called The Radiant Garden. This is the original home of many characters such as Kairi,Ansem the Wise, and the many human forms of Organization XIII’s members. Additionally, it is the world in which many of theFinal Fantasycharacters such asCid, Cloud, and Aerithcall home. It goes by the name “City of Light,” and consists of many people, shops, gardens, and a castle.
Its original Radiant Garden form can be explored fully inKingdom Hearts: Birth by Sleep. It is betweenBirth by Sleepand the original Kingdom Hearts game where the Radiant Garden gets transformed intoHollow Bastion. The catalyst to this drastic change is when anamnesiac Terra-Xehanortis brought on as Ansem’s apprentice.

Turning Into Hollow Bastion
About a year after the events ofBirth by Sleep, Xehanort and other apprentices (who later are part of Organization XIII) overthrow Ansem the Wise and turn The Raident Garden into Hollow Bastion. They did this by remodeling the castle and throwing Ansem into the Realm of Darkness.
The castle was turned into a half castle and half factory. Nine years prior to the originalKingdom Heartsgame, Maleficent comes to the world with her heartless and takes over. The residents who were able to escape the changed world mostly went to Transverse Town. This is also how Kairi ended up on Destiny Islands. Hollow Bastion boasts two places of importance, which are:

Changing Back To Radiant Garden
InKingdom Hearts 2, Hollow Bastion becomes the Radiant Garden again. This is because of Tron, who finds records of the Radiant Garden within a computer database. This gives rise to the Radiant Garden Restoration Committee (which was previously called the Hollow Bastion Restoration Committee), which seeks to kick all heartless out of the town and restore Hollow Bastion back to The Radiant Garden.
Within a year, the plan is successful. Cid even creates a defense system to destroy weak Heartless and distract the stronger ones until they can send a powerful committee member to take care of it. With the location being such an iconic one, it is hardly a surprise thatKingdom Heartsfans will be able to see itwhen Sora arrives inSuper Smash Bros. Ultimate.