Kayaba Akihiko did not know when he had first dreamt of that castle.
Aincrad.
A place to escape from the sadness of his family.
A place to be a hero or a villain.
Another world, beyond the scope of logic.
He had spent hours upon hours working at Argus, tweaking and fixing and creating.
A masterpiece of design, a world of its own, and the culmination of a dream.
He had started off the players and then begun to play himself, and the adventure had just ended.
He was standing in the sky with two players, who had just asked him why he did what he did.
He considered his life. A dead sister ("you were too late. The car... the car broke her and the gun finished her." Why couldn't you avoid that post office?), a poor family ("you must stay in the foster home for awhile, Akihiko." They never came back), and a single movie (a place to escape, imagine, dream... a floating castle). The Floating Castle had evolved into a World of Swords.
"I do not know."
And with that, Kayaba Akihiko released his ties to life. He faded away in the evening wind.
Alas, Aincrad. Farewell.