In the first Kingdom Hearts game only, Beast, from Beauty and the Beast, can join Sora, Donald and Goofy when they are in Hollow Bastion.
Radiant Garden was once a peaceful kingdom ruled by Ansem the Wise, a good king who dwelled in the castle Hollow Bastion, and populated by happy people. He was concerned with the spread of darkness in the worlds, and with his six apprentices (all now Organization XIII members) experimented on the depths of the human heart. Xehanort, his top apprentice from parts unknown, volunteered himself to be a test subject for these experiments. Unbeknownst to Ansem, his six apprentices gathered a great amount of test subjects and began dangerous experiments into the darkness of the heart. While Ansem ordered them to stop their experiments, Xehanort's experiments accidentally created the beings who he called 'Heartless' when he and the apprentices destroyed the hearts of their test subjects. Xehanort, behind his mentor's back, began creating artificial Heartless (known as "Emblems") in the basement beneath Hollow Bastion. Through further study, he uncovered the Keyhole of the Radiant Garden and discovered that behind it lay the world's heart.
On the night that Xehanort glimpsed the heart of this world, he also observed a meteor shower, the products of which were foreign like himself. He quickly concluded that travel between worlds must be possible, but did not know how to accomplish it. On the same night, Ansem was visited by King Mickey. On Mickey's advice, he decided to re-examine the data from the underground laboratory. He discovered many "Ansem Reports" that carried his name, written by his apprentice, but he had only written the one entitled Ansem Report 0. Ansem quickly learned that Xehnanort deduced that the goal of the Heartless was to find and consume the hearts of all worlds, just as they sought to consume the hearts of living beings.
Ansem's other disciples, Even (now Vexen), Ienzo (now Zexion), Braig (now Xigbar), Dilan (now Xaldin), and Elaeus (now Lexaeus) have ceased to be human, becoming Nobodies. Xehanort later exiled Ansem to the world of nothingness and took his name and place in Hollow Bastion. There, in the world of nothingness, Ansem took on the name DiZ, and became obsessed with revenge. Xehanort was obsessed with his newfound knowledge of darkness, hearts, other worlds, and the Keyblade. He cast off his body and disappeared to search for answers, plunging himself into the darkness without reservation and creating his own Heartless, "Ansem", and Nobody, Xemnas, in the process.
Hollow Bastion is also the home of several beloved Final Fantasy characters, such as Squall Leonhart (Leon), Cid Highwind, Aerith Gainsborough, Yuffie Kisaragi, Tifa Lockhart and Cloud Strife.
In Kingdom Hearts, Maleficent took control over the decayed remains of Hollow Bastion to serve as her base of operations. When Sora and company arrived to find Kairi, they encountered Riku who claims the Keyblade from him. Due to King Mickey's orders to follow the Keyblade bearer, Donald and Goofy unwillingly abandon Sora and follow Riku. Sora makes his way through the castle with the help of Beast, whose strong heart withstood his world's destruction and led him to this world. There, Donald and Goofy rejoin with Sora and battle Riku. Due to the strength of Sora's heart, the Keyblade comes back to him permanently. At the time, Maleficent was gathering the seven Princesses of Heart to open the Final Keyhole to Kingdom Hearts. Sora and company finally encounter Maleficent in battle and defeat her, but with the help of Xehanort's Heartless who has returned to the castle to witness the opening of the Final Keyhole, she becomes a huge dragon which the party defeats. Riku has now been possessed by Xehanort's Heartless, who still referenced himself as "Ansem, the Seeker of Darkness" at that time, and Sora had to battle a very powerful Riku without his companions. Before this battle, Xehanort's Heartless reveals to Sora that Kairi's heart remained sealed in him. As Destiny Islands was ripped apart, her heart left her body and used Sora's as a vessel. To unlock Kairi's heart, Sora used the Dark Keyblade that Riku possessed on himself. In doing so, Kairi's heart was freed, but his own heart was lost in the process, and he became a Heartless with Roxas born as a result. Because of Kairi, Sora regained his human body and because of this, Roxas, the Nobody of Sora, was born without memories of his former self due to Sora's short period of time as a Heartless. Now that the Final Keyhole was completed, Sora and company ventured inside, sealing the Final Keyhole once and for all.
Hollow Bastion returns in "Kingdom Hearts: Chain of Memories" where Maleficent plans to use both the Beast and Belle for her own ends.
Hollow Bastion plays the biggest role in Kingdom Hearts II, in a similar fashion to that of Traverse Town back in the original game. This time around in the world, Leon, Cid, Yuffie, Merlin, Aerith and others are attempting to rebuild the town. When Sora & Co. arrive, they are asked by Cid to help in the redevelopment.
However, later on in the game the world will come under attack by Organization XIII, which results in possibly the most dramatic and perhaps some of the best scenes in the game. During this battle, there are fights between Cloud & Sephiroth in addition to Sora & Co. facing off against a thousand Heartless as well as encounters with Maleficent and Organization XIII member No. 7, Saïx, and No. 1, Xemnas, the Nobody of Xehanort. Tifa, Cloud, Squall, Aerith, Stitch, and King Mickey also fight alongside Sora in the war scenes against the Heartless. This world also plays host to the destruction of Organization member No. 9, Demyx.
Hollow Bastion in Kingdom Hearts II also contains the entrances to two other worlds: Space Paranoids and the Hundred Acre Wood. Space Paranoids is accessed through the use of Ansem the Wise's computer, which the gang accesses hoping to find information on the Heartless and various other subjects
(Found on Wikipedia)
