First go to world 4 haunted house, next at mini Mario at the fifth/sixth room jump on the pipe to the next room and kick-jump on the three blocks, then kick-jump through the thin path until you reach the door, finally enter the door to the red flag.
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in world 1 at the tower there is a red flag that you need to get to unlock the cannon.
you can unlock a level with a pipe and cannon around it (i got it on a red flag on world 2 number 4 ) and finish that on a cheat flag( the yellow pipe above the other 1). this will unlock the cannon.
it is not stars it is a world
Get all 150 stars
get the cannon in the worlds