King Bowser's Castle is the final level in Super Mario World 2: Yoshi's Island (Secrets and Extras excluded).
The boss course starts out with a long gap which Yoshi must cross as the Yoshi Helicopter. Along the way, Yoshi must avoid the Skeleton Goonies, which crashing into will automatically result in Yoshi falling to his death. Eventually, Kamek will also fly in, trying to knock Yoshi out of the sky. If Yoshi makes it to the Yoshi Block (followed by a Middle Ring), Kamek will fly in for the last time in the area and look at Yoshi grimly.
The next room consists of a door roulette wheel. Doors, which are numbered 1, 2, 3 and 4 spin on the wheel, and Yoshi must pelt one with a Yoshi Egg, which will then drop to the floor and allow him to progress.
The first and the second doors contain generic obstacles, but the third and fourth doors are more notable.
In the third one, Tap-Tap the Golden He will chase Yoshi through a cavern that automatically scrolls. If Yoshi is lucky, he will be able to knock Red Nose back and into a pit with eggs, buying him some time. Eventually, Yoshi will reach a Warp Pipe to get him away from him.
The fourth door is an incredibly easy area. It consists only of moving platforms that hold Coins. The only possible threats are a few Gusties that fly through the stage and two Fly Guys at the end who carry 1-Ups.
The final room before the boss battle is a scrolling playroom. Kamek will appear, desperately trying to blast Yoshi with his Magic Wand. If Kamek hits any of the blocks in the level, they will turn into almost anything, from Shy Guys to Fangs, and possibly even Stars. If Yoshi makes it past Kamek's final shots, he will be awarded with a Middle Ring and a huge door that leads to Baby Bowser's room.
Kamek will greet Yoshi, demanding he hand over Baby Mario. Soon, however, Baby Bowser will awake, aggrivated that Kamek "woke" him. He will shove Kamek aside, noticing Yoshi and achknowledging him as a "gween donkey". He will attack Yoshi, trying to push Baby Mario away and ride Yoshi. Once Baby Bowser is defeated, Kamek will return, fully healed from his beating, and put his magic to its full power. Yoshi then finds himself in the now-destroyed room, and a giant Baby Bowser will appear in the background. Yoshi must nab Giant Yoshi Eggs from the Balloons that fly in, while avoiding both falling rocks that destroy the stadium and fireball blasts from Giant Baby Bowser. If Yoshi hits him enough times, Baby Bowser is returned to his normal self, and Kamek will fly off with him. Then, Yoshi will run up to a small area where the Stork and Baby Luigi will be trapped. The Stork can then make his delivery to the babies' parents, and the credits roll.