N's Castle and Champion Alder[]
About N's Castle belonging to Alder, that is not true. You battle him in the Champion's trainer, which has a warp link to the Hall of fame in Unova; however, N's Castle does not have a warp pad, unless you count the Grunt who returns you to the league PokeCenter. Besides, you can only visit the castle until the credits roll for the first time, afterwhich the building crumbles.