New Super Mario Bros./World 3-Ghost

This is the first Ghost House in the game, one of several decrepit mansions scattered across the worlds, filled with devious Boos and their confounding tricks. The mansion's dark magic prevents you from seeing your progress through the level, making it even easier to mislead you. This Ghost House in particular features rigged staircases that will have you pushing buttons and running about.

Level walkthrough
The starts off ominously calm, but you'll quickly notice something is off once you hit the first block. The power-ups in Ghost Houses are sent out at an arc, making it slightly harder to get them. Head right to get to your first main obstacle in this stage: an extremely steep wooden frame. You'll slide right off if you try to run up it, so you must find another way to head forward. Below and behind the slope is a P-Switch. Press it to temporarily cause steps to jut out of the slope, turning it into a staircase. Take this chance to run up to the top of the slope.

At the top is your first encounter with Boos. Boos are one of the recurring enemies in the Mario series, mostly confined to roaming the halls of Ghost Houses. They are rendered immobile when you're facing them, but will keep chasing you as long as your back is turned to them. There is no way to defeat Boos other than a shell, a Star or a Mega Mushroom, so they can be quite the nuisance. There is another slope to the right, which you can slide down to get some coins.

After a few jumps, you can spot a Splunkin. These pumpkin enemies take two stomps to defeat, the first angering them and causing them to speed up. Like the Boos, Splunkins are usually only found in Ghost Houses, and their movement patterns and relative harmlessness make them the haunted counterpart of the common Goomba. The door to the right of the first Splunkin leads to a small room. Hit the left block for a power-up, but avoid the right block, since it contains a Boo. Head out and go right to reach another temporary staircase. Before you run up this staircase, go under it and enter the door there. You will end up in a room with multiple Splunkins and a Star Coin. Jump off one of the Splunkins to get the coin.

Back outside, you must now begin scaling the slope. The P-Switch is located beneath the frame. Hit it, then jump to the moving platform. From there, quickly jump right to the staircase formed. Hop up the short steps until you see another staircase to your left. Jump to those steps and get to the top. Head left and fall down to obtain the next Star Coin. Fall back down and hit the P-Switch again. This time, climb the first set of steps all the way up. There seems to be a dead end past this staircase. Go right, sliding down a slope, and you will see a small space with a single brick block. Hit it to make a P-Switch pop out. Hit that P-Switch to make stairs appear again on the slopes. More importantly, it also causes a hidden door to appear at the top-left corner of the screen. Go right and climb up the closest ladder, then drop into the first gap to get to the door.

You'll end up in another large room. Go left and jump up the platforms to reach a roulette block. You can potentially get a Blue Shell power-up from this block, which is needed for the final Star Coin. Now head right to reach the final staircase. Press the P-Switch and start hopping up the slope step by step. Watch out for the Splunkins in your way as you do. At the top is the Ghost House exit. If you are Shell Mario, run past it and ram into the blocks to the right in your shell. This opens up a secret room in the wall, where the third Star Coin is located. Go back to the door and exit it. Outside, jump up two stone blocks to help yourself reach the top of the flagpole.

Star Coins

 * 1) Behind the door beside the second P-Switch is a room with the coin and some Splunkins. Bounce off a Splunkin to get it.
 * 2) After pressing the second P-Switch, go up the staircase and jump to the other staircase as soon as you can. Get to the top of that staircase for the Star Coin.
 * 3) You need to be Shell Mario for this. Before going into the exit door, make a Shell Dash to the right to break some blocks in the wall. The Star Coin can be found behind it.

Alternate exit
You can access this secret exit once you get to the third P-Switch, which is the one required to make the hidden door appear. While one did appear close by the moment you hit the switch, there is in fact another hidden door that the switch reveals. Once you hit the P-Switch, immediately run up the first set of stairs, then jump over the gap and fall down the stairs to the left. The hidden exit is to the immediate right of the second P-Switch. Outside, wall jump off the left set of blocks to get to the horizontal block platform. Jump from there to the red flagpole.

Going through this well-hidden back door takes you down a path to the World 3 Cannon. The cannon launches you high above sea level to the rocky peaks of World 6. Watch out for the hike in difficulty!