From the Super Mario Wiki, the Mario encyclopedia
Rotunda Castle is a large, pink-colored and 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, rooms in Rotunda Castle include a dungeon, kitchen, potion lab and Queen Rotunda's throne room. The castle also has a large courtyard in it, which is where the queen's wedding to Mario, or later Prince Pompadour, is held.