World 1-Fortress is the first fortress and contains the second Magic Whistle. Go to the right, the power up in the box is a Mushroom or Fire Flower. Keep going jumping over the lava pits and you'll reach the second power up. This is a Super Leaf. Stomp the Dry Bones then head right next to the door and run back left to fly up out of sight. Fly at top of the visible ceiling on the left part of the visible ceiling and when you can fly to the right press up to enter an unseen door. A chest is containing a Magic Whistle. Or go into the visible door and go between the space of the ceiling spikes. Then jump over the ledge and enter the door to fight Boom Boom, a Koopa that tries to prevent you from destroying the tower.
JPN's version difference:
In the Japan's Fortress, The ceiling with spikes had four more spikes on the right, those 4 spikes was removed in the USA version due to make it easier.