Hollow Bastion is an important place in Kingdom Hearts lore, but it’s where the first game becomes more serious, and Sora gains a glimpse into something much bigger than his journey to find his friends. It’s also the first non-Disney world since Traverse Town, and that alone gives you an idea that something big’s about to happen.
Of course, that's the fight with Riku, where all the tension and jealousy hinted at on Destiny Islands finally releases. Sora’s friends leave him for a time because of this fight, in a turn of events rather shocking the first time around. Suddenly, Sora is alone and friendless once more, all talk of loyalty and the ties that bind gone.
Of course, the situation doesn’t last, and he gets his friends back, but then you see what happened to all the princesses. The sight has more impact for Disney fans, naturally, but it’s an important step forward in the games’ overarching plot, something you don’t really get the significance of until playing the other games.
And then there’s Hollow Bastion’s crowning glory, the fight with Maleficent’s dragon form. Suddenly, the terrifying beast from your childhood cartoon days manifests in front of you, and it’s so.damn.tough to beat, yet so satisfying once it’s finished.