From the Super Mario Wiki
Rotunda Castle is a large, pink-colored and oddly-bulging castle that is the home of Queen Rotunda, the ruler of Rotunda Land; Rotunda Castle appeared in The Super Mario Bros. Super Show! episode "Love 'Em and Leave 'Em".
Located seemingly in the center of Rotunda Land atop a mountain, Rotunda Castle has many rooms in it, including a dungeon, kitchen, potion lab and the throne room and of Queen Rotunda. Rotunda Castle also has a large courtyard in it.