Diablo 4 - and in fact the entire Diablo series - takes place on an entire planet, rather than the flat plane that the games' isometric perspective makes it appear they're set on.
In a tweet, Harrison Pink, senior quest designer on Diablo 4, offers to share a "tiny lore nugget," stating for the record that "Sanctuary is a planet. I don't know if we've ever officially stated it or not, but it's 100% a spherical celestial body with at least one moon, not a flat plane with edges."
