Kirby 64: The Crystal Shards/Pop Star 3

Castle
This castle belongs to King DeDeDe, with nine sections.

The first section is outside the castle, where you can simply advance until you reach the drawbridge. The second is a climb up the stairs. The third requires riding a platform, where you can inhale a few random flying enemies, and the fourth is a climb up onto an outer wall.

When you reach the outer wall, head left if you've taken any damage for a Maximum Tomato. On this wall, head to the right - in order to collect a shard, you will want to obtain a dual-cutter from some of the enemies in this area. Once you have dual-cutter, jump down the exit on the right. Only a simple sandwich is to the right of this drop.

After the drop, you can use the dual-cutter to reach the first shard. Adeleine will be in the next room, drawing a 1-up for Kirby.

Boss: King DeDeDe
The first time you reach the end of this stage, you will see King DeDeDe becoming possessed after trying to claim the crystal shard. In this combat, he is practically feeble if you have any significant attack.

King DeDeDe starts combat by simply swinging a mallet at Kirby. Inhale the star and shoot it back at him. After three hits, he will start to fly.

A total of six hits will knock down King DeDeDe, allowing you to retrieve the shard, and have him follow as a companion.

1st shard
On the outerwall, inhale two Sirkibbles for their cutter power. When you exit this section, you can break open the green block with the dual-cutter ability.

2nd shard
After seeing Adeleine and proceeding into the circular room, jump over the entrance to reach the second shard.

3rd shard
Reward for defeating King DeDeDe.