New Super Mario Bros./World 2-Castle

Bowser Jr.'s desert stronghold is quite unique, as it has an outdoor courtyard in addition to the usual indoor hallways. Spiked Balls of various sizes is his main weapon against you, and you'll have to jump or duck a lot of these as you traverse the castle. Bowser Jr. has resurrected an ancient creature to act as the boss of this castle, and how you defeat this undead terror will determine where you'll go next.

Level walkthrough
The action starts immediately as soon as you enter the castle. A Giant Spiked Ball will come tumbling down directly above you. Wait until it bumps into a low wall ahead and falls into a pit before going forward. After jumping over two pits, smaller Spiked Balls will come rolling from the right. These can be easily evaded with a well-timed jump. Past two Spiked Balls, you will see a third one bouncing between both walls in a small valley. Your tactic here is the same, jumping whenever the ball gets close.

Up ahead is another Giant Spiked Ball, one that also bounces back-and-forth between two walls. You have to use the rope attached nearby to swing across. If you can swing high enough, you can reach a bunch of coins as well as a 1-Up Block. After jumping over another pit, the hallway will start to narrow and Spiked Balls will once again roll at you. There is less space to jump over them, but there are indents in the ground where you are safe from them. You must duck into these indents if you're Super Mario or larger. After two indents, you must jump on to a platform instead to avoid the Spiked Ball. After that, wall jump up a vertical shaft and quickly fall through the gap along the upper walkway to avoid the next Spiked Ball, letting you collect the first Star Coin.

Backtrack a bit and fall down the shaft, then continue to the right. One more Spiked Ball will roll by as you jump to the door that leads outside. Once you're outdoors, you'll quickly activate the midway checkpoint. There is a new danger here: Bullet Bills. They are launched from Bill Blasters and travel in a straight line forward. However, you can prevent a blaster from firing if you're standing right next to it. After the first blaster, you will drop into a valley with multiple blasters. Stand on the lowest blaster to minimize the amount of Bullet Bills shot at you, then jump on a Bullet Bill launched from the left blaster. The Bullet Bill should give you some lift, popping you up high enough to reach the ledge with the next Star Coin.

After the valley is a pretty narrow hallway with a Bill Blaster at the end. Get ready to duck as stray Bullet Bills will fly by regularly. Once you reach the blaster, you appear to be in a dead end. The switch here will make the ground slant downward, letting you progress forward while avoiding a Spiked Ball rolling around the newly-created pit. However, you can also jump over the blaster and offscreen. Get a running start and jump to the next section of ceiling to skip a short part of the stage and get some coins. If you took the conventional path forward, you must swing over another Giant Spiked Ball, which takes you to the same platform you end up after falling from the ceiling.

There's a tiny walkway here, as well as a riskier regular corridor below. One of the blocks on this platform contains a Mini Mushroom. It is required to get some secrets in this stage, and if you manage to keep it until you defeat the boss, something good will happen! Go right through either walkway and jump over the gap. A Spiked Ball will roll down, which you must dodge by jumping or hiding in the tiny recess as Mini Mario. Jump up a step and evade some more Spiked Balls. If you're mini, you can go through the small walkway to access a small pocket with the third Star Coin. There's a block containing a Super Mushroom if you want to revert to normal. As you go further right, you come across another dead end. Pressing the switch makes the ground sink like before, revealing the double doors to the boss of World 2.

Boss: Mummipokey
Deep in the crypts of World 2 lies the tomb of an ancient Pokey pharaoh. Bowser Jr. has awakened this embalmed king and commanded it to fight against you! The Mummipokey is nothing like the cheery Pokeys you're encountered before. You can't knock off individual sections of its body. The Mummipokey will dig through the ground, emerging from it from time-to-time. He will shoot out from the sand at varying heights, and the time he stays on the surface depends on how tall it is. It can also spit a gooey projectile at you. To defeat it, you must simply stomp on its head three times, but it will sometimes be too tall to leap over. You can also throw a ton of fireballs at it, or use Shell Dashes. Once Mummipokey is defeated, you will get the key to the next world!

If you defeated the Mummipokey normally, Bowser Jr. will take Peach to World 3, a sunny beach with lots of submerged areas. However, he will reconsider his choice if you manage to triumph against the Mummipokey as Mini Mario. While fighting a boss in Mini form is usually inadvisable, it is actually quite worthwhile here. Like in a normal battle, you have to stomp the Mummipokey in the head three time to win, but you can only damage it with ground pounds. Mini Mario's floaty jumps can also make it hard to maneuver yourself on to the Mummipokey's head. If you find the straightforward method too challenging, you can also pack a Mini Mushroom in reserve and damage Mummipokey first, then turn Mini to deal the final hit. As long as you are Mini Mario when you defeat Mummipokey, Bowser Jr. will escape to the poisonous swamps of World 4 instead.

Star Coins

 * 1) Can be seen near the end of the indoor section. Wall jump up and fall into the gap to get it. Watch out for the nearby Spiked Ball.
 * 2) Up on a ledge early in the outdoor area. Use a Bullet Bill to give you the height boost to reach it.
 * 3) In plain sight in the outdoor section, near a bunch of Spiked Balls. A Mini Mushroom is needed to collect it.