Bowser Jr. and his father awaits in this massive fortress, with Princess Peach locked up in the throne room. As the king of the Koopas, Bowser can afford to install a bunch of fancy obstacles inside his ultimate castle. From mechanisms that flip entire areas upside-down, to winding hallways that seem to continue onward infinitely, reaching him requires both skill and intelligence. Bowser himself is not going to stay a skeleton for long, and you can rest assured that he will put up quite a fight in this final showdown.
Before the checkpoint
Bowser's castle starts out with a standard lava section. After crossing several bridges where Podoboos shoot out from below, you will have to contend with more Podoboos while crossing a series of one-tile wide gaps. Look for a safe opening in the Podoboo patterns and dash across. Jump across some more gaps, being mindful of the remaining Podoboos, and go through the door. Hit the right block in this small room for a power-up, then hit the block in the center to make a switch appear. Once you hit the switch, the entire room will flip upside-down. Now you can go through the door that was on the ceiling.
In this long room, you can immediately notice the primary threat here: Rocket Engines. Most of the Rocket Engines in this room activate periodically, while a few are always active. Ignore any alternate pathways for now and just concentrate on moving to the right. Patiently wait for a Rocket Engine to extinguish before moving forward, and do not risk running past a Rocket Engine about to activate just to save some time. Another danger in this room is the two spike pits on the ground. They aren't very wide, but the placement of the Rocket Engines forces you to cross the pits quickly and precisely. Enter the door at the right end of the room.
There is a Super Thwomp in this room. You must quickly jump over the block on the floor as the Super Thwomp is rising. The question-mark block high up contains a power-up. Meanwhile, the third brick block from the Super Thwomp contains a switch. Stomp it to flip the room around again. You must evade the Super Thwomp again, which you can either do by jumping to the platform on the left or falling down and simply running underneath. Go through the door at the end.
This room certainly seems familiar. Indeed, you are back in the room with the Rocket Engines, except now you're upside-down and traversing the area on what used to be the ceiling. You must evade the Rocket Engines once again and travel all the way to the left, past a fairly wide spike pit. Hit the brick block here to make a switch pop out. Hitting it doesn't cause any flipping to occur, but it does make a moving platform spawn beyond the left wall. Backtrack to the middle of the room until you can see a ledge above with a permanently activated Rocket Engine. Jump on to the ledge and move right. There's another permanent Rocket Engine here. If you're too tall, you must do a dashing slide to move past the fire unscathed, and enter the door beyond.
In this new room, you must immediately get past three Thwomps in a row. You've dealed with these enemies plenty of times by now; just wait for the right moment and proceed forward. After you jump on to the ledge to the right of the last Thwomp, you can choose to go for the first Star Coin right now. When the nearby Thwomp slams down, jump over it to the left. Run down the hallway to the end and you get the Star Coin as your reward. Go back to the right and drop down. The lone block right of the spike pit contains a power-up.
Go left and drop down the steps. You should see several pairs of brick blocks. Hit the right block of the middle pair for the room-flipping switch. As you've seen of the Super Thwomp from before, flipping the stage also alters the Thwomp's positions. As you climb up to the top, you will have to dodge several more Thwomps, with less safe space around them. If you've missed the first Star Coin or chose not to get it. Drop down to your left once you get past the first Thwomp. Dodge the Thwomp here and go left to grab the Star Coin you missed. When you're done with the room, head right, dodging another Thwomp and a spike pit to reach the door.
Upon exiting the room with the Thwomps, you find yourself in a smaller room with a spike pit too wide to jump across, as well as a door nearby. If you did not hit the switch back in the upside-down Rocket Engines area, your only choice is to enter the room to your right. If you did hit the switch, there will be a moving platform over the spike pit. Use it to reach a door to the left. The door takes you to a room where you can get the second Star Coin. Whichever room you entered from the previous area, there should be a single door that takes you to the second section of this confusing castle.
After the checkpoint
Head right and you'll past the checkpoint. You're at the midway point of the castle, which means there is no more large scale flipping. Instead, this stage uses a classic looping trick first utilized by Bowser way back in Super Mario Bros.. Jump over the lava to the platform and get the power-up inside the block, then jump to the following platform. From this raised part of the ground, you can see and jump to three levels of platforms. You must choose a path to go down, and if you chose wrong, you will end up back at the start of the fork in the way forward. The only way to figure out to correct order is through trial and error, although you do get audio cues whenever you choose the right or wrong path.
For this first choice, you want to go down the lowest path. Dash across the floor to cross the small gaps in the path. After that, you must jump on to several platforms above the lava, dodge one Fire Bar and then arrive at the next multiple path choice. There are only two choices here, and the top path is the correct choice. Immediately afterward, you are confronted with three more choices. If you get this choice wrong, you will loop back to the start of the second path selection. Go down the middle lane to successfully clear all of Bowser's tricks and challenges. You will see the final Star Coin in the game near the end of the hallway. You need to be tall Mario to break the brick blocks with your ground pound, but this makes it hard to squeeze through the thin gap. You must run and jump from the floor to the right, over the lava, onto the platform and duck to slide under the gap in order to grab the coin. Jump out and go into the double doors to fight the final boss.
Boss: Bowser and Bowser Jr.
You reach the battle arena, just in time to see Bowser Jr. tossing Bowser's skeleton into a cauldron of dark magic. Using arcane Koopa sorcery, Bowser is revived back into his former self, and you'll be double teamed by both Bowser Jr. and Bowser! Bowser Jr. uses his shell-throwing attack from the tower boss battles. Meanwhile, Bowser has some new attacks. He can breath fireballs and make huge leaps as always, but now he can also breath blue fire that shrinks you down to mini size!
Your ultimate goal here is to hit the large skull switch behind Bowser, but since he is so huge, you must wait for an opportunity to get past him. With Bowser's fireballs and Bowser Jr's shells, it is very hard to find a safe opening to get past father and son. The recommended strategy is to focus on Bowser Jr. first. You've faced him using his shell tossing strategies multiple times, so you should be able to do him in with a bit of effort. With his son down, Bowser becomes angry and will breath fireballs more frequently. Now, all you have to do is to dodge Bowser's attacks and wait for him to make a big leap, then run under him to the switch. Once you press it, Bowser will join his son in the abyss below, and Princess Peach will be saved!
While watching the credits, you can see an album of every level you have cleared in the past. Tap the letters in the credits to hear different sound effects from the game. After viewing the credits, you will return to the menu screen, but not before getting a hint to use , and to play as Luigi.
- There are two chances to get this Star Coin in the room with the regular-sized Thwomps. Before flipping the area, you must jump on to the ceiling to the left of the highest Thwomp and run left to grab it. Alternatively, after flipping the room upside-down, you must drop down to the left and dodge the leftmost Thwomp to get it.
- In the room with the Rocket Engines, you must hit the leftmost brick block after flipping the room to make a switch pop out, which causes a platform to appear in an adjacent room. After you pass the room with the Thwomps, you will end up in an area with a wide spike pit. If you pressed the switch before, you can use that platform to jump to the door. Inside is the Star Coin.
- The Star Coin is near the boss doors. You need to ground pound the brick blocks blocking you from getting it. Then, run left from the ground to the right and fall to the platform with the coin. Duck and use your momentum to slide through the small gap to grab the Star Coin.
Even though you have completed the main game, some changes will occur on the overworld. Every Toad House on the map is now permanently available, which means you can enter any one of them for as many power-ups or lives as you want. You can also now save on the map whenever you wish. If you have any Star Coins left to collect, both of these features will make it much easier to finish your hunt. You should also spend your Star Coins on any signs that still block your path for total completion.
Additionally, when you return to World 1, there will now be a purple Toad House above World 1-1. Inside, you can purchase backgrounds for your lower screen, each costing 20 Star Coins. Even if you don't want a fresh coat of paint for your interface, buying all the backgrounds is required to get 100% completion, so it's well worth your time to do so.