The Other Castle Grounds is a scrapped level (discovered in the July 25th 2020 Leak) that could have served as a tutorial level, letting players adjust to the controls before taking off to the Castle Grounds. While this stage was relatively large, there exists little in it, being a flat plane of grass with a few trees. It is possible it was removed due to this large size, and the tutorial was instead squeezed into the Castle Grounds. Due to the nature of the Other Castle Grounds, it also could have been an edited test level.
Remnants[]
Contrary to the design of the castle grounds, the lake to the right of Mario’s initial starting point does not have a hill behind it. This blank area would supposedly connect to the Other Castle Grounds. However, players might not access the Other Castle Grounds, likely because of an invisible wall.
Reimplementation[]
Recently, ROM-hackers have managed to insert the Other Castle Grounds back into the game, even making a pipe that leads back to the Castle Grounds. Some areas of the map also don't have collision or textures, however the latter have simply been replaced with grass textures. This implies that the level was unfinished, or was potentially corrupted.
This area may be accessible in standard copies via means of personalization.