With Byleth making into Smash Ultimate, her stage is Garreg Mach Monastery which is the main area in Fire Emblem: Three Houses. For which you spend the most time in the game. It serves as a school and a church.
The stage is pretty flat and simple. It just moves you around the academy. You still plenty of cameos like Dimitri and Ingrid.
Edelgard and Dorothea from the Black Eagles.
Claude and Hilda from the Golden Deer.
And of course, Lady Rhea, Seteth, and Flayn.
While it’s far from the best stage, I do like it. It’s very true to its home game.