If I had to take a stab, I'm guessing he was referring to the Cornerstone of Light, since neither he, Goofy, nor Mickey would be there to protect it. You see, originally Disney Castle was supposed to be a world in the first KH, and evidence suggests that the story would have been the same as it was in KH2, with Maleficent trying to acquire the castle by corrupting the Cornerstone. But when DC eventually got scrapped, they basically repurposed that world's storyline into KH2. I have no evidence to support this, it's just my shot-in-the-dark guess.