That's a pretty inconvenient location for a castle
Mario Circuit is the Wii U's Mario Kart stage, or one of them, at least. This stage is from Mario Kart 8, the only racing game Nintendo will ever make for the Wii U. Mario Circuit is a mobius strip track meant to show off Mario Kart 8's brand new and completely original anti-gravity mechanic. The Smash 4 stage too shows this off by having the platform thing stop on the walls and ceiling of the track. And once again, those Shy Guy racers from Brawl's Mario Circuit are back. Did you really think anyone wants to remember Pink Gold Peach?