World 1 is the first world in New Super Mario Bros. Wii, containing six levels, a fortress, a castle and a boss, three special stages - accessible by entering the Mushroom Houses - as well as housing Princess Peach's Castle. There are three special stages, two reds and one gold; the two reds occurring upon completion of World's 1-2 and 1-4, and the Gold Mushroom House occurring upon completion of World 1-3.