The Grand Mario Hotel is the location from a one off episode called The Grand Mario Hotel, its located somewhere on the SMG4 Multiverse since Marios from across the dimensions get to reside at it.
Appereance
it looks like a big wooden chest with a door on the front protected by a rainbow kind of forcefield, it has many floors on the inside it also counts with numerous rooms and hallways, there’s lots of bathrooms, a big dinning room and what it appears to a game room.