Kirby begins his adventure to find the thieves that stole his mouth-watering shortcake in Prism Plains, which is a peaceful area consisting of flat plains which trail off into the mountains and through a dense forest. King Dedede's castle is located on a cliff in the heart of the plains. Once Kirby makes it onto the cliffs, he takes a Warp Star to King Dedede's castle. Kirby makes it through the bottom half of the castle, and then goes to the tower to fight the king. After defeating King Dedede, a hole in the castle roof made by the escaping Squeaks is visible on the stage select screen. The castle bears a major resemblance to its anime incarnation, and even has the monster transporter in the throne room. The transporter only exists as a graphical element, and isn't actually used.
The boss of Prism Plains is King Dedede, who is fought inside his castle.