After you have beaten the Elite Four, N's Castle will spring out of the ground and give you access. This castle is a multi-levelled castle with a variety of characters who will give you the history of the Team Plasma leader; N. This area can only be accessed once so make sure to get all of the items from there on your first trip. In Pokémon Black 2 & White 2, the ruins of the castle can be accessed via Victory Road.