after awhile you can get out through the pipe spot.but first you have to help the moles with bowser
You get hit by a laser. then you fall down and the pipe yard inside bowser opens up and you get out through a pipe
keep hittin him with a hammer and special attacks
You can walk over there, or you can take the purple swirly circle thing and press "Bowser Castle" and you'll be in Bowser's Castle.
Unfortunatly, unless part of the storyline, this doesnt happen.