The Grand Mario Hotel is the location from a one off episode called The Grand Mario Hotel. It's located somewhere on the Multiverse since Marios from across the dimensions get to reside at. It also appears in R64: a Dose of Dr. Mario, during the scene where Dr. Mario takes a pill that was lying on the floor.
It looks like Booster's Tower with a door on the front protected by a rainbow kind of force field. It has many floors on the inside. It also comes with numerous rooms and hallways, lots of bathrooms, a big dining room and what appears to a game room.