The Old Mansion in Twilight Town, going in in-game chronology
We don't know where Ven was born, the earliest we ever see him is training under Master Xehanort
A decade passes, more than enough time for a pleasant Mansion to became old, abandoned, and presumed haunted.
When Roxas comes into being he pops up in Twilight Town, right outside the mansion
In CoM Ventus creates a Twilight Town from the memories on the other side of Sora's heart. Now in Days Roxas never goes there more than about 10 times (as far as plot goes, extra heartless etc. not withstanding) and most of these haven't yet occurred. Since Sora and Ven share a connection it's reasonable to consider Ventus was talking about memories Sora has because of Ven. And the only area of importance in this illusory floor is the Mansion.
In KHII's Twilight Town there are odd KH related symbols in various places
Obviously none of this is conclusive, or even probably important, but it is interesting in my opinion, as it explains why the games with it seem to have it be such an important point.
The Old Mansion in Twilight Town, going in in-game chronology
Obviously none of this is conclusive, or even probably important, but it is interesting in my opinion, as it explains why the games with it seem to have it be such an important point.