The Top Secret Area can be accessed by unlocking the secret exit in the Donut Ghost House. The Top Secret Area contains 5 ? blocks. The two on the left contain Fire Flowers while the two on the right have Feathers. The middle ? block contains Yoshi. As with all other boxes in the game, hitting the Feather or Flower boxes while small will yield a Mushroom and hitting the Yoshi box when you have a Yoshi will yield a 1-up mushroom. This is a great area to accumulate on 1-ups and power-ups, especially when preparing for difficult levels.
When you leave this level in 2-Player mode, it's still your turn!