Location from pokemon-unova by
Team Plasma's hidden fortress — a medieval castle that erupts from beneath Opelucid via a mechanical lift, where N was raised and Ghetsis operates.
N's Castle is the emotional engine of the climax. It's the home where a child was raised in a constructed reality; it's the fortress from which Ghetsis runs his political coup. When it rises from beneath the ground in the final episodes of the arc, the visual is one of the anime's most memorable. Inside, the contrast is brutal — N's room is a nursery, Ghetsis's throne room is a war command center, and the lower levels hold the Kyurem control chamber.
A dark stone castle with black-and-gold banners, seven spires, a great hall with a throne at the center. Interior chambers are sterile and lab-like — the room where N was raised is a single isolation room painted bright cheerful colors at child-height, with a mounted screen as the only window to the outside.