What Happned..

[Final772]

I think the hole thing just came down to rubble and when the worlds were restored again the castle got restored.....

[XShadowSoraX]

No,
Only one of the towers which was part of Org XIII's castle fell down the rest of it was still in tact so i think Melificent has took over it.

[MarluxiaNo11]

I think that what XShadowSoraX says is correct for the dragon only took one of the towers off so im going with ShadowSora's Theory

[DarkCreature2]

The Caslte of TWTNW will probably come back to huant us in KH3. Maleficient is still at large remember, she and Pete tried to fend off the Heartless while Sora and the gang went to take down Xemnas. We still don't know what exactly happened to her.

[Organization_42]

Honestly, I hope Maleficent took over the castle, because I love the World That Never Was and wouldn't mind seeing it return. But the whole Xemnas-throwing-buildings and their little ship getting sucked into that black-hole-thing makes me think he destroyed the city. Oh well, we'll see more of it in 358/2 Days.

For everyone talking about worlds being restored, well, that was KH1, not KH2. None of the worlds got destroyed in KH2, the Organization was simply putting them in danger by releasing Heartless.

Originally Posted by Musical Muffin

Maybe a really nice young couple hoping to start a family moved in and after they moved in, they told all their friends about it and then all their friends moved with them and they started a whole new world.
Called 'Yaiiburg.' (Yay-burg) XD

Wife: We need a nice neighborhood to settle down in...
Husband: I know! How about a city of eternal night, with dangerous Heartless and Nobodies lurking around?!
Wife: Well...
Husband: We'd live in a castle!
Wife: PERFECT!