Hollow Bastion is the world over which Ansem the Wise reigned as king, followed by Ansem, Seeker of Darkness, who overthrew him. It was the home of Leon, Cloud, Yuffie, Aerith, Cid, and others. As we know, Ansem became obsessed with the Darkness after studying Heartless and began artificially multiplying them under the castle. The castle was deserted and reinhabited by Maleficent and Riku.
This world is originally known as the Radiant Garden in Kingdom Hearts: Birth by Sleep, before being taken over by Heartless and having only the castle remaining. It is referred to as Hollow Bastion in the first two games. However, in Kingdom Hearts II it is once again renamed the Radiant Garden.
 Kingdom Hearts
In Kingdom Hearts, Sora arrived at this world and met Riku again. This time, Riku took the Keyblade, being the true master of it. Sora was left behind with Beast, who was looking for Belle and trying to save her. Beast protected the now vulnerable Sora as they solved the puzzles of the Waterways in order to access the front gate of Hollow Bastion's castle.
When they entered, they found Riku in the antechamber. Sora prepared to fight again, but felt confident in his ability to win, because of the strength of his heart and of his friends. Riku, annoyed by Sora's returned and slipping into control of the darkness, attempted to destroy him with a blast of dark energy. Goofy, who, along with Donald, was following the new key bearer, intervened and saved Sora from being destroyed. The Keyblade then returned to Sora, for he had a stronger heart, and Sora defeated Riku. Riku fled into the castle, and Sora was left to solve his way through the corridors. He defeated Maleficent after reaching the Castle Chapel. Wounded, she began staggering away to in an unknown location. Sora followed her through the portal of darkness, and found Riku, who was now possessed by Ansem. He was also brandishing a keyblade of his own that had the power to unlock people's hearts. He stabbed Maleficent's heart, and unlocked her inner darkness. She transformed into a dragon, and attack Sora and his friends. After an arduous battle, the Dragon Maleficent fell and decayed into green flames.
Sora returned through the portal and made his way to where the Princesses of Heart were kept. He found Kairi lying on the floor there, unconscious. Ansem revealed that the Door To Darkness cannot be opened without the last heart of the Princesses, Kairi's. Her heart had been residing in Sora's the whole time. Ansem challenged Sora in a fight for her heart. It was a painstaking duel, but Sora prevailed. Ansem dropped his keyblade and vanished. Sora then took it and stabbed his own heart. This released Kairi's heart and returned it to her body and restored the Princesses of Heart as well. Unfortunately for Sora, it also unlocked the darkness in his heart and he vanished. Ansem returned, this time in his actual, physical appearance. Riku, in a desperate attempt for control, inhibited Ansem from attacking them and told Kairi, Donald, Goofy to escape. They immediately began making their way out of the castle. Sora reappeared as a Heartless and followed them through the castle. He caught up with them in the antechamber. Donald initially wanted to fight him, but Kairi sensed Sora's presence. Desperate to have him back, she clung to him. This reawakened Sora's light within his darkness and restored him to his original self. The four of them fled Hollow Bastion and took Kairi to safety. Later, Sora returned with Donald and Goofy to seal the Keyhole in Hollow Bastion.
 Kingdom Hearts II
In Kingdom Hearts II, Sora arrives to find Leon, Yuffie, Aerith, Cid, and Merlin in league together, trying to reconstruct their home world. The group is known as the Hollow Bastion Restoration Committee. When Sora first gets there, he mostly reacquaints with the group and joins the committee. Soon after, leaves after dealing with a small threat of attacking Nobodies and seeing the Organization standing only to vanish very quickly. The second visit is much more eventful. Sora starts looking for Leon. He is in the lower levels of the castle (which looks nothing like the castle from KH I). The Hollow Bastion Restoration Committee has been searching the castle for secret chambers and such to find any useful information that Ansem hid away. Sora actually reaches Ansem's Study, which has been thoroughly searched. On the wall, Sora sees a portrait of Xehanort, although, he still believes him to be the true Ansem at this time. Tifa walks in on the scene, she's apparently searching too. She rummages around and even takes down the large portrait, but she finds nothing.
 The Computer World and Restoration
After she leaves, Leon is seen against the wall. He reveals a secret passageway that leads to a very large computer. They cannot access it, so they go back to the Study to look for clues. The wall behind where the portrait hung reveals these clues for the passcode. King Mickey takes this moment to reappear and explains the real identity of the man in the portrait. Sora and his friends then access the computer. In doing so, they are accidentally beamed in to the computer, taking you to another world, Space Paranoids. Mickey is about to explain what happened to Riku, but trouble brews. An army of Heartless attack the town. With the help of Mickey and all of our Final Fantasy heroes, Sora wades into battle. On his way to the battle, Demyx appears, and Sora slays him in a frustrating fight. Then Sora makes his way to the battlefield, which is swarming with 1000 Heartless. He defeats them, and the threat of Heartless for now is halted. The third and final visit takes you back to the computer world. The MCP (Master Control Program) has taken over the town's defense system and released cyber Heartless. Sora goes in and stops this threat, thus putting an end to all calamity in Hollow Bastion, and restoring it to its original title, Radiant Garden. Which is why there's all kinds of waterfalls everywhere if you haven't noticed.
Related ThreadsSora vs Demyx Hollow Bastion - last post by @ Apr 1, 2007
Keyholes in Hollow Bastion - last post by @ Jun 23, 2006
hollow bastion - last post by @ May 26, 2006
How do you get to Hollow Bastion? - last post by @ Jan 20, 2006
Hollow bastion, world of chaos? - last post by @ Jul 30, 2005