The first Kingdom Hearts game begins with the disappearance of King Mickey from Disney Castle. He leaves to investigate the growing Heartless threat and leaves Donald with instructions that eventually lead him to join the story's protagonist, Sora, on his quest. While the plot of Kingdom Hearts unravels from there, the history of Disney Castle and its royal family—King Mickey and Queen Minnie—remains relatively unexplained throughout the plot of subsequent games.
To complicate matters further, Disney Castle happens to be one of the few locations in the Kingdom Hearts franchise that does not draw inspiration directly from Disney works. This means that fans of the series cannot rely on outside knowledge for answers. Instead, the only way to answer the lingering questions about Disney Castle and its inhabitants is to comb through the oftentimes complicated Kingdom Hearts timeline.
Kingdom Hearts: The Lore of Disney Castle
While Disney Castle makes frequent appearances throughout Kingdom Hearts, it's not until Kingdom Hearts 2 that fans finally get a glimpse into the history, or lack thereof, of the castle. As players traverse the Timeless River, one of the many worlds in Kingdom Hearts 2, they are shown a time before Disney Castle was built. While this does give players a loose grasp on the timeline of Disney Castle, it's unclear exactly how much time separates the events of Timeless River and the present day in Kingdom Hearts 2. It does seem to suggest that Disney Castle was constructed specifically for King Mickey and Queen Minnie sometime thereafter, meaning that Disney Castle itself has almost no history that predates the royal couple.
Still, Disney Castle holds an extremely important role in the Kingdom Hearts franchise since it is the holding place of an ancient relic called the Cornerstone of Light. Though it is temporarily disrupted during Kingdom Hearts 2 and Kingdom Hearts 3D: Dream Drop Distance, the Cornerstone of Light casts a barrier against the most powerful Heartless villains in Kingdom Hearts. As such, Disney Castle acts as a central hub for Sora and his companions.
Kingdom Hearts: How Did Mickey Become King?
As with the history of Disney Castle itself, the history of King Mickey and Queen Minnie is also not clear-cut. A possible scenario is that King Mickey and Queen Minnie's ascension to the throne is more democratic than our real-world equivalents. It's possible that Mickey, after having trained with Yen Sid, becomes a sort of de facto leader in the Timeless River world through his deeds against the Heartless. From there, Mickey travels and meets Minnie, and together they are more or less elected, by Donald, Goofy, and the other inhabitants, to become the royalty of Disney Castle. Unfortunately, the events of Kingdom Hearts 3D: Dream Drop Distance complicate matters more, but offer an alternative explanation about how the royal family of Disney Castle came to be.
Kingdom Hearts 3D: Dream Drop Distance introduces Sleeping Worlds. In one of the worlds, the Country of the Musketeers, Mickey, along with Sora, Donald, and Goofy, is depicted as defending the world against Dream Eaters and various foes. In the Country of Musketeers, however, Minnie already exists as Princess Minnie. At this point in the Kingdom Hearts story, then, Minnie is a royal and Mickey is not, leading to the possibility that he married into royalty.
If the complications of Sleeping Worlds are ignored, it's possible that this Mickey is the same Mickey as the one from Timeless River, and the events of Kingdom Hearts 3D: Dream Drop Distance are merely a recounting of his earliest adventures. If that's the case, then Mickey appears to have first met Princess Minnie in the Country of the Musketeers, after which they commissioned Disney Castle. Then the pair either formed a new royal line or had Mickey join Minnie's preexisting one, leading to the existence of the royal family as seen in the original Kingdom Hearts.