Square Enix's Kingdom Hearts franchise stands out for setting its epic long-form narrative in a universe that blends Final Fantasy and Disney properties. Many of the games take place across Disney Worlds inspired by classics ranging from Steamboat Willie to Frozen, but there are also plenty of places unique to the series that build upon its internal lore while offering crossovers like Scrooge McDuck and FF7's Aerith found together in Hollow Bastion.
One of the more interesting original locales in Kingdom Hearts is Castle Oblivion, a place Sora, Donald, and Goofy first encounter in Kingdom Hearts: Chain of Memories. Chain of Memories bridges the gap between Kingdom Hearts 1 and 2, but its mysterious castle setting opens up numerous mysteries and plot threads - some of which would not be resolved until 2019's Kingdom Hearts 3, 15 years after the Game Boy Advance title. Castle Oblivion is deeply woven into the story of Kingdom Hearts in ways that are hard to appreciate without playing numerous side games, so it helps to take things back to the beginning with current knowledge.
The Origins of Castle Oblivion
Those with a passing understanding of Kingdom Hearts should know it has a deep obsession with philosophical and spiritual subjects relating to people's hearts and the universal balance between good and evil. There exists a Realm of Light and a Realm of Darkness in Kingdom Hearts, and one world between the two was the Land of Departure. In Kingdom Hearts: Birth by Sleep, Keyblade Master Eraqus is the steward of this world and begins a Mark of Mastery exam for his students Terra and Aqua; giving them the chance to become Keyblade Masters.
Birth by Sleep follows this exam for Terra and Aqua, with their friend Ventus also being a playable character. However, the test is overtaken by Eraqus' old friend Xehanort - the ultimate bad guy for the series. He kills Eraqus, takes over Terra's body to continue his work, and renders Ventus comatose in an attempt to forge the legendary X-blade (Chi-blade). Terra's spirit is lost in a suit of armor called the Lingering Will, and Aqua brings Ventus to a Land of Departure overcome by darkness. She hides him in the Chamber of Waking before transforming the land into Castle Oblivion to keep him safe.
Castle Oblivion in Kingdom Hearts: Chain of Memories
Some time after Castle Oblivion is created, it is discovered by Organization 13 - an evil group with ties to Xehanort and comprised of Nobodies, the husks left when a person loses their heart. Kairi's Nobody, Namine, is coerced by the Organization to rewrite Sora's memories as he climbs to the top of Castle Oblivion; skills reduced to that of a card game, with new decks able to be built using memories he gains by revisiting levels from the original Kingdom Hearts. Marluxia, the appointed head of Castle Oblivion, hopes to use Sora to usurp Organization 13 leader Xemnas: Xehanort's Nobody.
This scheme fails, in-part thanks to infighting by Organization members like Axel, and after Marluxia is defeated Namine agrees to restore Sora, Donald, and Goofy's memories. They are put in stasis, explaining Sora's absence and eventual awakening at the start of Kingdom Hearts 2. Meanwhile, Riku has to make his way through Castle Oblivion's basement, dealing with his own inner turmoil and other Organization members. By the end he begins on the road back from the darkness he succumbed to in Kingdom Hearts, priming his arc for the rest of the series.
The End of Castle Oblivion
Castle Oblivion plays a role in other games, including revelations from Kingdom Hearts 358/2 that it was the birthplace of Xion - who helps turn Sora's Nobody Roxas and Axel against Organization 13. However, its next big role comes in Kingdom Hearts 3 when Aqua is rescued from the Realm of Darkness and transforms the castle back into the Land of Departure so they can retrieve Ventus. It turns out Castle Oblivion is named such because anyone who tried seeking the Chamber of Waking for themselves would keep wandering into oblivion.
The lore in Kingdom Hearts is deep and complex, confusing without factoring in multiple games and spin-offs all being relevant but only accessible from varying consoles. Even a basic rundown of Castle Oblivion can't cover every detail, such as the connections between Ventus and Sora that require the series' lead protagonist to rescue Aqua's friend. Luckily, with the whole Kingdom Hearts franchise available on PC it should be easier than ever for fans and newcomers alike to dig deep into everything before Kingdom Hearts 4 inevitable drops.