How do I get on top of the castle to get to Bowser?

Bowser's not on top of the castle, Yoshi is. You get on top of the castle by getting all 120 stars. Once you have all 120 stars, the cannon by the water opens up. Yoshi says thanks for playin the game, gives you 99 lives and jumps away into a waterfall...yay...

First, second or third time?

LOL you don't mean the top of the castle, do you?

You mean the top of the highest tower.

You need to keep on going up the castle by collecting more and more stars. First you get 20 to be able to beat the first bowser, then you use that key to go to the basement and beat bowser there using 32 stars.

Then use that key to go through the locked door on the first level and go to the second level (snowman's land, big/small world, wet and dry world).

Then you get 50 stars there to go to the third level, then you get I think it's 60 stars to open the doors to the endless stairs.

Once you get to the endless stairs, you need to get 70/75 stars (not sure which) and the stairs aren't endless anymore.

Then you can beat the final bowser.

Happy gaming!

You need 120 stars to open the cannon in outside the castle. It's Yoshi not Bowser though. Good luck.

You mean Yoshi and you get to Yoshie by getting 120 stars.