OW MAN! This is an awesome, adrenaline-pumping version of a famous Super Mario World work.
My only criticism is your title...This isn't exactly the Bowser music. This is the music for the castle/fortress itself. If you want to know what Bowser's music is, pop in your Super Mario Kart cartridge and select Bowser's Castle 1-4. That's the music that plays when you fight Bowser in Super Mario World as well.
You've remixed the fortress/castle music, but this could easily have been seen as a secondary Bowser theme. This actually happens to be the best piece of music in the whole game, and I've got to wonder if it's the best in the series. Way to go seanofah4!