N's Castle

N's Castle is a location that appears after you defeat the Elite Four. You encounter Reshiram or Zekrom here depending on the version of the game being played (Reshiram: Pokémon Black; Zekrom: Pokémon White). To go there, you must beat all of the Elite Four members of Unova and then walk into the blue circle around the statue and press A. It will lower and there will be a set of stairs. N's Castle is at the top. You will see N talking to Alder.

When you walk up the black steps, you will see the sages for Team Plasma, and then all of the Gym Leaders (excluding Cilan, Cress, and Chili. Bianca later explains that by the time she got to Striaton City to get them everything at N's Castle was over.), Alder, and Cheren confront the Sages. To the left of the Sages, you will see a flight of steps to lead to the next part of N's Castle.

After 3 flights of stairs, Team Plasma's Shadow Triad will appear and tell you that N is in that room with Zekrom/Reshiram. When you enter, you will see a 3D scene of Zekrom/Reshiram (The one that appears will be under the control of N). Then Reshiram/Zekrom appears with yet another 3D scene. Once you catch it, you will have the ability to switch out one of your Pokémon in your Party for Zekrom/Reshiram and put the other Pokémon in the PC Box. Afterward, N heals your Pokémon and then battles you. When you defeat N, Ghetsis appears, and you are healed again before he battles you. Once you defeat Ghetsis, the game will end with him being taken into custody. N later releases his Reshiram/Zekrom after Ghetsis is taken away.

After the game ends, if you battle the Elite Four again, N's Castle is inaccessible, Alder's room is a far as you can go.