Sadly (I do know how you feel, you know) the only way open the cannon outside the castle is to collect all 150 stars. When you have done this, just walk up to it and it should be open. If you have an Action Replay card, however, there is almost certainly a cheat you can use to get the cannon before this.
Collect 120 stars
Collect all 120 stars and go to the cannon outside. If you are playing super Mario 64 ds get all 150 stars and go to the cannon outside.
Get all 150 stars
If you mean the outside cannon you have to collect 120 stars. If you're talking about the ones in the courses you have to talk to a bob-bob buddy (pink bob-bob).
You cannon play multiplayer mini games in Super Mario 64 DS. Are you sure you don't mean New Super Mario Brothers? It has multiplayer mini games.
Collect 120 stars
You gotta get all of the stars, then the cannon outside will open up.
You must collect all 150 stars, then it will automatically open.
To get on the castle roof of the castle
Collect all 120 stars and go to the cannon outside. If you are playing super Mario 64 ds get all 150 stars and go to the cannon outside.
Get all 150 stars
either use a cannon or a wing cap
Waluigi is not in the game at all.
You will need to collect all 150 stars and the cannon in front of the castle should open and then all you have to do is go in the cannon and blast off.
If you mean the outside cannon you have to collect 120 stars. If you're talking about the ones in the courses you have to talk to a bob-bob buddy (pink bob-bob).
You cannon play multiplayer mini games in Super Mario 64 DS. Are you sure you don't mean New Super Mario Brothers? It has multiplayer mini games.
You can use the cannon to shoot up to the top of the castle. Yoshi is there in the Nintendo 64 version. After you talk to him, you will have 100 lives.