So I was reading through the Secret Ansem Reports from KH2, and I saw in what I think was the 13th, AtW says: "How did Xehanort manage to open the door that appeared in the basement of my castle…?". Now, I don't remember the exact wording of it, but in one of the Ansem Reports from KH1, Xehanort writes that he opened the door in the basement of the castle. But how? He couldn't have just walked up to it and opened it. He had to be able to unlock the lock somehow. He had to have a key. This brings me around to the "Terra-is-Xehanort" theory, and I know some of you don't like it, but I really think this could lock the theory for me. I mean, they both look similar and Terra weilds a keyblade. Granted Xehanort didn't at the time, he may have been able to at one time, and using that memory his heart (that he had before submitting to darkness) was able to unlock the door.

Yes, many people have suggested this.
But there's an error.

The door to the heart of Radiant Garden wasn't locked at that time. None of the keyholes were until the events of KH1 (when Sora went around locking them). So, theoretically, Xehanort could have simply opened the door.

But there is other evidence that backs this idea. The door to the heart of worlds appears when a keyblade is nearby. Hint, hint.

I thought Xehanort turned up at AtW's castle with all his memories vanished? If he did do what you believe, how come he never used a Keyblade in Kingdom Hearts I when you fight him?

Yes, that Xehanort is, but that was Master Xehanort

I think the door was left open at the end of BBS, which is how Xehanort got there in the first place. He just forgot to close it, or that he had used it to begin with.

Plasmos wrote:I think the door was left open at the end of BBS, which is how Xehanort got there in the first place. He just forgot to close it, or that he had used it to begin with.

I don't think he "forgot" to close it, it just so happen that someone had a key to reopen it, like KINGDOM HEARTS.