Crocodile Isle is not only the home of the Kremlings, but is also King K. Rool's hideout.
This stage is very similar Melee's Icicle Mountain and Brawl's Rumble Falls. You start out at the base of Crocodile Isle and work your way up towards the top. The screen doesn't roll straight up, but rather diagonally around the fortress. Nothing really happens until you get closer to the top. If you touch fortress' big teeth, IT HURTS!!! You may even get KO'ed.
But when you get to the top, a plane comes out of nowhere. Hop on the plane and it will take you back down to the bottom to start the cycle over again. You can fight while on the plane, but the plane takes off for the bottom 2 seconds after it appears. If you're not on the plane when it takes off, you get KO'ed.