Way back in the original Super Mario Bros., it was implied that the castles you fought the fake Bowsers in were Mushroom Kingdom property, and the Koopa Troop was holding them as part of their military conquest. People have always wondered why were they so dark and lava-filled, things that were shown in later games to be Dark Land architectural norms. As of this game, we can see that Bowser used his dark magic to give all the castles an Extreme Makevover, Koopa Edition.
This is awesome. People wonder where Bowser gets the lava, or how volcanic elements could appear just about anywhere? People forget Bowser does know dark magic. He just likes a "flaming doom" decor.
Why is it that Bowser can move so freely in this game's final boss sequence, whereas in the previous games, it seemed like everything was a monumental effort to do? One must remember he has always been indoors before now, in constructs made for smaller people...but here, he's actually outside and free of obstructions.
At first, Peach's Castle seems to be breaking the Edible Theme Naming rule. Then you think about the fruit...
It always seemed weird that Bowser Jr jumps off Peach's Castle once you beat Bowser. And then I played Sonic Lost World, and the Giant Zavok fight reminded me of Bowser's giant mechanism in the Mario & Luigi series (Bowser's Inside Story and Dream Team). And then I realised: It's a biological function. Junior must have thought he had the ability to do the same. Which is Fridge Horror at the same time, because either a) Bowser Jr died because of a chance or b) there is a giant, presumably angry Koopa that Mario completely ignored!