Hollow Bastion

Hollow Bastion is a world from Kingdom Hearts and Kingdom Hearts: Chain of Memories.

Overview
Hollow Bastion is a colossal castle that towers above the rest of its world, the Radiant Garden. At one point, it was the only remaining part of the world, along with the waterways beneath it. Hence, the world itself was known as "Hollow Bastion" in Kingdom Hearts and Kingdom Hearts: Chain of Memories. For most of Kingdom Hearts II, it was named this as well. However, it regains its original name by the end of the game.

Unlike the rest of the Radiant Garden, which poses as Final Fantasy's world, Hollow Bastion has no relation to Final Fantasy whatsoever.

The boss of Hollow Bastion (on the first visit) is Maleficent, though Riku is also battled. On the second visit, the boss is the Behemoth.

History
Hollow Bastion was a glorious castle owned by Ansem the Wise. Much of the history of the castle is unknown, other than that Ansem ruled the Radiant Garden from it. However, eventually Xehanort took over, and remodeled the castle to include multiple machinery. The result was a twisted bastion; half beautiful castle, half distorted factory. Maleficent, during her reign of the castle, made additional changes as well. It is probable that she either created or remodeled the castle's keep, which has decorations and models of roses and her thorned plants.

In Kingdom Hearts
Maleficent used Hollow Bastion as a base to lead her horde of Disney Villains and the Heartless. When Sora, Donald and Goofy reached Hollow Bastion they found Riku attacking the Beast. Sora's keyblade was taken by Riku and because Donald and Goofy had to follow the key, they left Sora for Riku. Sora went inside the castle, after unlocking the main door and confronted Riku and got the Keyblade back, thanks to Goofy and Donald's loyalty to him. Riku fled the scene, and was confronted by Xehanort's Heartless, who promised Riku the power to defeat Sora, and save Kairi. In his frustration over his loss to Sora, Riku gave into the darkness, and was possessed, but was given a Dark Keyblade that was made by the princess' hearts. Sora and friends defeated Maleficent, who was then transformed into a large Dragon by Riku, who stabbed the Dark Keyblade into her which unlocked her heart. Sora and company were still able to defeat her, and Maleficent seemingly died. Sora and company then moved to the main hall, where Sora found Kairi and was challenged by Riku. Sora found out that Kairi's heart had been inside him all along, and that she would only awaken if Sora unlocked his heart. And so, after he defeated Riku, he stabbed himself with the Dark Keyblade, unlocking his heart, transforming himsef into a Heartless, returning the hearts of the princess (including Kairi) to them, and creating two Nobodies; Roxas and Naminé. Riku saved Kairi, Donald and Goofy's lives, by regaining control of his body briefly, and telling Kairi to run. Kairi and company then ran into Sora, as a Shadow, and because she was using Sora's heart as well as her own, she was able to restore Sora to his original body. The four of them then fled Hollow Bastion, but Sora, Donald, Goofy would return to the main hall, defeat the Behemoth, and seal the Keyhole. Sora also has his first encounter with Xemnas and any Organization XIII member here; he fights Xemnas in the same room he fought Maleficent, and after some cryptic messages, Xemnas leaves, revealing in Kingdom Hearts II that he was testing Sora.

In Kingdom Hearts II
Leon, Yuffie, Aerith, and Cloud have started the Hollow Bastion Restoration committee; an organization attempting to rebuild the world that the four grew up on. Sora first arrives here after meeting with Yen Sid in Twilight Town and has his first encounter with Organization XIII here. After defending the town from Nobodies, Sora heads out to search for King Mickey and Riku. When he returns, he finally meets the King, and learns that the King can't tell him about Riku. Sora also enters the world of Space Paranoids for the first time. After defeating the giant Heartless army Maleficent has conjured, he kills Demyx, and meets Saïx and Axel who both tell him that they cannot help him. He also learns of the Organization's plans. When he returns yet a third time, he travels into Space Paranoids, and when he defeats the MCP, he learns from the data Tron gathered the true name of the town; Radiant Garden.

Trivia

 * Part of the original music for Hollow Bastion involved a segment from the Sitar Concerto No. 2
 * Kairi was born in Hollow Bastion, where she lived with her grandma. The current whereabouts of the grandmother are unknown.
 * Hollow Bastion poses almost like a world for two Disney movies. One was Sleeping Beauty, while the other was Beauty and the Beast.  However, it is mentioned that both actual worlds were destroyed, and the latter is visited in Kingdom Hearts II.