Nothing has been mentioned of the castle since Power Ponies, so there is no way of knowing until further information is released. If any news in this regard is revealed, it would quickly be posted on Equestria Daily.
Yes, Cadance will appear in the My Little Pony Friendship is magic episode Twilight's kingdom.
Yes, the shining armor will appear in the My Little Pony Friendship is Magic episode twilight's kingdom.
the episode is called "Bridel Gossip"
Yes, the My Little Pony Friendship is magic episode Twilight's kingdom is involved in the rainbow power.
It has not been revealed who the villain in Twilight's Kingdom will be.
Yes. The Villain in Twilight's Kingdom has been revealed to be named Tirek.
No information has been revealed as to who the villain will be.
No information has been revealed as to who the villain will be.
It is not announced when promos will start. Such things are usually not announced.
Season 1, Episode 1: Friendship is Magic
In all likelihood, yes. It has been mentioned that Twilight will focus her efforts on unlocking the mystery of the box, and no new episode leading up to the finale will center on Twilight.
Most likely. Since this is a children's show, a happy ending is to be expected.