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As they enter the castle, all Mario, Link, Kirby, Pit and Yoshi finds is a collapsed ceiling and a conspicuous hole in the wall, leading to an underground cavern. With King Dedede nowhere in sight, the group decides to venture in to the treacherous cave.
As soon as you run right you will trigger a falling boulder. Stop immediately and wait for it to roll down the slope before following it. After that is the first instance of a descending ceiling. Move out of the way to avoid getting crushed by it. Go right a bit more and fight some Goombas, then climb up the ladderr at the end. You will be locked in battle with waves of Bowser's forces. The small space makes it difficult to avoid attacks, but you can use the koopa shells to clear off many foes at once.
Drop down the hole to a lower level, then keep going right to find a vertical shaft with a descending ceiling. Fall down the shaft quickly to avoid getting crushed by it, then go right. You will now have to contend with a rising floor. Jump on to the various platforms and onto a ledge in the left wall, then jump up into the hallway. Go right to find another vertical shaft with a descending ceiling.
Walk close to the shaft until you trigger the ceiling, but wait in the hallway. Once the ceiling completely descends, you will find a path leading to an alcove in the opposite wall that contains a door. Enter it, then quickly get the contents of the blue and gold item boxes inside. Exit the room before the two walls around you crush you. Back outside, you must wait until the ceiling fully descends again to move back to the previous hallway, then wait for it to return to its original position to go down the shaft.
Drop down the platforms, being mindful of the bottomless pit at the end of it as you do. Don't miss the gold item box on the lowest platform in the shaft. Go right and climb up the ladder. In the following hallway, moving walls will continuously move to the left, which can knock you into one of the pits along the floor. You must pass through the walls by fitting through the gaps in the moving walls. At the end of the hallway, you can find the door to the next area.
The room you end up in is small but dangerous. Grab the key and go to the right, carefully avoiding the falling spikes and the fire pits. Toss the key into the locked gate to unlock it, then go through the door beyond.
Go right down the hallway until you reach a cloud of purple gas. This gas will slowly damage you if you stay within it, so make sure to avoid it whenever you encounter it. Here, you can attack the switch on the ground to clear the gas temporarily, letting you pass unharmed. At the end of the hallway is a vertical shaft. The falling platforms might make it difficult to ascend to the next hallway.
On the upper level, go left and jump on to the large platform with an indent to start a forced battle with a mix of Bowser's personal troops and Subspace forces. Keep going left until you reach another shaft. Your climb will be a bit harder, as there are not only enemies around the area, but also more poison gas. Attack the switch to temporarily deactivate the gas, then quickly climb to the top.
On the next hallway, go right and attack the switch below another cloud of poison gas. You have to run back left and jump to the top path when you hit the button, so it might be harder to clear the cloud in time, especially if you're caught by the Bucculus in the ground. Keep going right to reach yet another gas cloud. Hitting the switch to clear it will also trigger a falling ceiling, so quickly run to the right to avoid getting crushed.
Go right and you will find another poison gas cloud, but the button is within the cloud itself. If you want to avoid damage, you can attack it remotely if your character has a projectile. However, as soon as you step into the chamber where the gas was, you will be locked in battle with some Giant Goombas. Defeat them quickly before the gas returns, then go right.
You will enter a large room without a floor. You must jump across the floating platforms, including a Ferris wheel mechanism, to reach the other side. As you ascend up the slope, you will find a falling boulder at the end of the hallway. Jump into one of the gaps in the ceiling to avoid it as it rolls pass you, then attack the button to make a door appear on a ledge.
Enter the door before another boulder falls. Inside, a blue item box rests on a platform that crumbles if you stand on it for too long. At the right end of the room is a gold item box. Return to the door and go through it. One final Giant Goomba stands between you and the golden door. Go through the door to end the stage.
The heroes exit the cave, but instead of King Dedede they find Bowser with the trophy of a princess. The group attempts to get him to drop the trophy. An arrow is fired at Bowser, causing him to lose his footing and fall off the cliff...into his Koopa Clown Car. Mario, Link, Pit and Yoshi watch helplessly as Bowser makes his escape, while Kirby checks out the brooch that fell off the trophy.
On the other side of the cave, a Subspace Bomb is detonated, pulling the castle, the cave, King Dedede and his two trophies into the void. Meanwhile, Ganondorf receives some orders from a powerful higher-up: Master Hand, guardian of the Smash universe. Ganondorf accepts the new orders, but he is secretly plotting something on his own...