This is the final level towards facing Bowser. Ingame, it's actually called "Front Door".
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When you go into the Front Door of Bowser's Castle, you get a set of doors numbered 1 through 4. Enter one of them and clear its area to reach another set of doors numbered 5 through 8. What doors do you like to use?

This video is specifically edited to show you all 8 doors and what rooms you'll have to go through in them. I did this by recording video clips of 1 & 5, 2 & 6, 3 & 7, and 4 & 8, then splitting them up and reattaching them into the correct numerical order.

After the 8th door's challenge, you go through a dark room that you can soon light up with a red "?" Block. This is the first room you reach when you enter this castle through the Back Door on the map.

Throughout this whole level, you'll find Mechakoopas. They only appear in this level, but you can pick them up after you stomp on them.
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At the very end, Mario reaches the top of Bowser's Castle. The great and terrible Koopa King slowly descends into the battlefield using his Koopa Clown Car, all while the storm of the valley continues its roars. The final battle begins!

First off, in his Clown Car, Bowser will fly left and right across the screen. If you're not gonna show off, stay low and avoid the propeller blades under his car. He'll soon throw & sic Mechakoopas at you. Beat them and throw them straight up, all while hoping that two of them will hit Bowser.

After the two hits, Bowser will fly away from the screen then extremely close to it. The Clown Car WILL hit you if you don't get out of its way while it's doing this.

Hellfire will rain on the field, then the Clown Car comes back. Princess Peach pops out screaming for help! She throws a Super Mushroom to help you out before Bowser appears again.
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This time, the Clown Car will home in on you, and drop huge bowling balls at you. This makes it a bit harder to hit Bowser with two more Mechakoopas, but once it's done, every thing from the flying away to Peach throwing a Mushroom will happen again.
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For the 3rd & final phase of the match, the Koopa Clown Car gets seriously angry and bounces around the field. It'll stop for a moment to drop more bowling balls and Mechkoopas, but situate yourself correctly so you don't get hit by them, or crushed by the Clown Car.

Just try and hit two more Mechakoopas at Bowser and his Clown Car will lose control. He'll drop the Princess and spiral away.
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Once again, Mario has saved Princess Peach Toadstool, to which she rewards him with a kiss. Together, both of them, Mario's new buddy, Yoshi, and all of the trapped yellow Yoshi eggs from the Koopalings, head back to Yoshi's House to start the vacation Mario & Peach wanted this whole time.

After a bunch of traveling during the credits roll, Mario & friends make it back to Yoshi's House, where there are Red, Yellow, & Blue Yoshis waiting for them. A miracle then happens as all of the yellow eggs Mario bought back finally hatched into different colors of baby Yoshis.

The 4 adult Yoshis celebrated, while Mario & Peach posed & waved for a victory. THANK YOU.

What follows after that is the cast of every single enemy in the whole game, including the Koopalings and Bowser himself.

THE END. Luigi somehow makes it into the art at the very end whether your game was set to 2-Players or not.
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The game may be over at this point, but my playthrough of it isn't! Stay tuned for videos of this game's Special stages; the true final levels in the whole game!

Since I now have a partnership, I can make custom thumbnails now, like the one you probably saw for this video. You'll be seeing those more often from me.