From the moment you enter the castle, you'll already notice that the lava below is slowly rising, so you'll have to not only avoid burners, Dry Bones, Dry Goombas, but also run quicker as the lava is rising. When you get to the snake blocks, it gets easier; all you have to do now is avoid some moving burners and you'll eventually get to a door, at which point the lava will also stop rising. Enter the door. After taking three steps into the new area, Larry, Iggy, Ludwig, Lemmy, Wendy, Roy, and Morton will appear in their Koopa Clown Car, and try to stop you from getting to their dad. You'll now have to run thought a group of treadmills and platforms. You also have to avoid the Koopalings' only attack of turning you to stone (you can tell when the attack is coming by looking at the Koopa Clown Car's eyes, which will glow). This attack can only be avoided by hiding behind one of the many pillars in this part of the castle. After a few jumps, the Koopalings will fly off-screen and another door will appear. Go into it and you'll come face to face with the King of the Koopas himself.
Boss: Bowser[edit]
As soon as you reach the hall, Bowser will breathe fireballs at you until you reach the end of the hall. After you reach him, the battle begins. First, Bowser will move toward you, then he'll do an attack against you (or do nothing). After that, he'll move backwards, then do another attack. He'll then repeat the same cycle over again. The attacks Bowser has are: breathing a fireball at you, breathing multiple fireballs at you, doing a small leap and breathing a fireball at you, throwing hammers at you, and doing a giant leap in the air.
To defeat him, either wait for him to do a giant leap and then run past him or shoot fireballs at him until he gets fried (requires a fire flower to do this). Then, hit the "!" switch on the other side. Unfortunately, your victory is short-lived as Princess Peach (who is still in her cage) will be pulled away, up off-screen. Follow her by jumping on the three platforms that appear; guess who also appears after you go up off-screen? The Koopalings then laugh at Mario, and use their magic wands to make Bowser so big that he knocks Larry, Iggy, Lemmy, Ludwig, Wendy, Roy, and Morton off-screen. Now, the true battle begins!
This time, you must jump on floating platforms to avoid getting burned by the rising lava. To make things extra spicy, Bowser will attempt to claw you by slashing his claws on the row of platforms you're on. As soon as Bowser raises his claws, he'll lock on the platform you're currently on. Immediately jump onto a different column of platforms. While Bowser's claws don't hurt you, they destroy the platforms, potentially making you fall to your death (the lava). Keep doing this until the screen stops scrolling and you jump to the very top of the screen. You are now on a safe platform, safe from the lava for a little bit. Afterwards, Bowser will attempt to breathe flames on the floor from left to right. Go near the right wall, and with a running start, jump over the flames. After that, the lava rises again and the floating platforms appear again. Do the same thing, except this time, there will be Bowser statues breathing fireballs at you. Watch your step! Also, on the second safe platform with no lava, Bowser will breathe flames on the floor twice before the lava rises again (first, from left to right, then right to left). Again, do the same thing that you did on the first no-lava platform, except when Bowser breathes flames the second time, start near the left wall before jumping (remember to have a running start before jumping). Repeat until you reach the giant "!" switch. Hit the switch! Bowser will then start sinking and will eventually collapse to his death. Congratulations! You rescue Princess Peach and rid of Bowser entirely (for now, at least)! You win and now you can Save instead of Quick Saving!