"Those portals are openings into the Corridors of Darkness, they allow travel as you have noted to other worlds beyond this one in a considerably easier and faster fashion that would be otherwise possible. However they are unpredictable and dangerous to those whom still possess a heart due to the fact they are formed from Darkness." Xemnas explained, turning away from the castle's exit to walk back toward Crooked Ascension.
"Let us leave Nothing's Call and return to Twilight's View, there is considerably more for you to see if you are still interested in seeing more of the castle."