get a certain number of stars 14 I think then go to the main room. You will see a light beam.Look directly at it then once in the level fly to the shortest tower and hit the switch. p.s you can only do this with mario.
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To unlock them you after collecting 12 stars look up in the castle circle and activate the red switch to unlock all ? blocks.
collect all the red star rings then beat all the levels in the sonic simulator
in world 1 at the tower there is a red flag that you need to get to unlock the cannon.
All you do is get a mini mushroom go into 2-4. Go all the way down to the water, go left through the pipe and you should see the red flag which should unlock the mushroom.
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.