Super Mario World/Back Door

Your reward for making your way through the Valley Fortress? A shortcut in Bowser's Castle. This level is like the ../Front Door/, but starts at the dark room. The boss battle with Bowser and the end credits both remain unchanged.

Dark Room
It's dark at first, but hit the question mark block to turn on the light above you. This area is the only area in the game where you see Ninjis. Right before the door to Bowser is a checkpoint. The Midway Gate is exclusive to just the Back Door. Go through the doors, where you will find Bowser in a flying contraption.

Bowser!!!
This part is not timed. Mario appears on the rooftop and Bowser makes an entrance in his Koopa Clown Copter.

As is befitting of the Final Boss, he is the longest to fight, and requires more strategy than "jump on him three times to win".

Be careful of the propeller! He'll swoop back and forth across the screen; just stand in a corner to avoid him. Once he stops, he'll throw 2 Mecha Koopas over the side of his flying machine. Jump onto one of the Mecha Koopas, then throw it up so that it hits Bowser from up above. Do this again, and Bowser will fly off. Avoid the flames that drop from the sky. Peach will appear and throw you a mushroom right before Bowser reappears.
 * Phase 1

Your second bout with him will be slightly more difficult, as he drops gigantic black marbles that will flatten you if you don't jump over them. You can also Spin Jump off them. Once again he'll throw 2 Mecha Koopas out of his machine. Toss them up at him again, and after hitting him twice, dodge the flames as you wait for a second mushroom from Peach.
 * Phase 2

When Bowser reappears, he's MAD, and so is his Koopa Clown Car. This time, he'll try to land his propeller on you. If you can dodge him long enough, he'll throw 2 more Mecha Koopas down. If Bowser lands on a stunned Mecha Koopa it'll be knocked off screen, so pick it up quickly and throw it back up at him. After hitting him twice more, he'll go flying off out of control into the darkness, and Peach will appear.
 * Phase 3

Congratulations! You've beaten Bowser!