From Bulbapedia, the community-driven Pokémon encyclopedia.
N's Castle is the location where the player can catch Zekrom or Reshiram, depending on whether they are playing Pokémon Black or White.
Appearance
N's Castle appears to be built into the Pokémon League building. The castle looks like a trapizium without the bottom.
Major events
One of the events that takes place in N's Castle is catching the legendary mascot of either Pokémon Black or White. The other event that takes place here is the final battle with N, who will use the version mascot of the opposite version. In this battle, N also uses the event exclusive Pokémon, Zoroark.
Trainers
N
Geechisu