This is how I originally imagined Part 4 beginning.
There are a few reasons I like what I wound up going with better, but one is simply that we got to see Hope making her own effort to get good at the game. Had I just gone with this, it could have felt like she was just trying to manage on inherited Pandora knowledge of the game.
At some inevitable point, the room's going to get at least partially explained in-comic. This is me writing this, after all.
For now, I'm just going to say it's a magic room, and:
Please note that these are claims I am making in this commentary, and they could get contradicted later if I wind up having different ideas (an example of one reasons why I say "it's not canon unless it's in the comic.")
To be fair, the 2nd and 3rd points are unlikely to change. What might change is the first one. Maybe I'll want a smaller surface to work. Maybe I'll think doors in the floor are cool. Maybe I'll decide to make it an even three feet. Only future me knows for sure, and I'm not them yet.