On the top floor of this humongous house lies Rikki, the third golden coin holder. While he should be easy to defeat, the mountain tunnels leading to his residence has mysteriously shrunk you down to the size of a bug. You will need to dive into an anthill, make your way through the house's plumbing system, climb the house's fireplace and conquer the dusty attic to reach Rikki. Size is no excuse for giving up, so go ahead and go for the golden coin!
Level 1: The anthill[edit]
In this level, prepare to face ants, a lot of ants. In addition to the Antotto you might have faced back in Tree Zone, you'll have to face other varieties like Chikuntos with spiked backs, Gorontos that can dig up rocks and chuck them at you, and even Dokanto, which can launch projectiles at you using the cannon strapped to its back. The platforming is going to get harder from here on out, in fact you'll have to face the constant threat of falling into toxic instant-death water and can even get denied from accessing places due to a slight mistake.
There is at least one section where you're forced to go beneath a low wall on a moving platform. While you can make it across without much effort as small mario, you'll need to crouch when passing through, or else you'll fall off the platform and to your demise. The blocks that can only be destroyed by fireballs return in this level, and is even required to access a few secrets. At the area with five question mark blocks, you can go to the lower level, crouching to avoid the spikes, and reach a block that contains a fire flower and a pipe next to it. Do not jump at the pipe while hugging the wall. Doing so will reveal a hidden block that will prevent you from entering unless you become small, but since the pipe entrance needs to be cleared with fireballs, it means you'll have no way to access it. On the other side is a chamber full of coins and question mark blocks. You'll need to ride the last moving platform to reach the bell at the end.
There is a secret exit in this level, one that will open up a devious shortcut and a secret level. If you can't find it yourself, check the spoiler below.
Secret Level: A midair shortcut[edit]
A level exists high above the air that can only be reached by skilled spelunkers who have managed to find an entrance to it above an anthill. This stage has forced scrolling and lots of deadly drops, so you'll need to think fast and act faster to avoid getting crushed or falling. If you're quick enough, you can net yourself some rewards. After the staircase of blocks, you can drill kick down the column of breakable blocks to reach a chamber full of coins (you need to do that if you're not small mario anyways, or else you'll be crushed). You can also use fireballs to open up another room and clear the blocks within to get a 1-Up.
If you manage to clear the level, you will open up a shortcut to the final stage of the zone where you can challenge Rikki the rat. You can still go back and clear the other levels if you like.
Level 2: Extreme plumbing[edit]
You might have made it out of the anthill, but you are still outside the house. It's foolish to try and enter through a giant closed door, so you'll have to infiltrate the house through the building's pipelines. Mario frequently move through pipes, but none of them are ever this massive. You have encountered most of the enemies in this level except the Cheep Cheeps, which are fish that swim in the water and can only be taken out normally with a fireball.
A fire flower is handy in this level, since not only will you be wading through a lot of water where you'll be defenseless without fireballs, you also need them to take out the nasty pirahna plants and venus fire traps that reside in the pipes and pots throughout the level. Fortunately, this level has a smaller focus on platforming and more on introducing swimming controls and surviving against a bunch of enemies.
If you have a super mushroom or a better power-up, you can get a Star chain going and potentially a few 1-Ups. In the small room just before the checkpoint which contains a pirahna plant, there is a block that will give you a Star. As soon as you get it, make your way through the level as fast as you can, trying to take out as many enemies along the way. At the first room immediately after the checkpoint, you can get another Star by using the drill kick on the box as long as you're still invincible, which lets you extend the period of invincibility and net you more 1-Ups! You can easily earn 2 or 3 extra lives if you're quick enough.
The last section of the level may seem straightforward, but it hides a nice secret. After you come out of the pipe, at the last body of water with the lone Cheep Cheep, jump up while hugging the left wall to reveal a hidden block. Keep jumping and revealing blocks until you reach a platform high above the ground, which will give you a ton of coins! You can jump down from the platform aligned to the center of the coin arrow at the end to land right on the ending bell, but don't worry if you miss, as there are a bunch of blocks that you can use to reach it with ease.
Level 3: Conveyor belts and fire[edit]
You've made it into the house, but Rikki's lair is on the second floor. While you can take the stairs, getting there through the chimney is a much faster, albeit slightly more dangerous, way. However, you'll need to make you way across the kitchen, along a deadly conveyor belt and over a burning stove to reach it. You're still not safe after reaching the fireplace, as you'll also have to jump from ledge to ledge inside the chimney without falling, or else you'll be toast, literally! This level features more traditional Mario enemies, though the Be return. You might also encounter the Honebon, a nearly unkillable skeleton fish.
There are some well-hidden secrets in this level. The third pipe along the conveyor belt doesn't contain a pirahna plant, but it does lead to a chamber full of coins. You can also get more coins at the end of that section by revealing some hidden blocks to the right of the pipes that will let you reach some coins located higher up. In the section with the koopas, you can enter a blocked off chamber by destroying the blocks with fireballs, which will net you a money bag inside the sole block in the room. Additionally, the block below the pipe will reveal a Star to let you get some extra lives by taking out the koopas, though if you fall in one of the gaps between the pipes you will be unable to get out other than using the pipes next to you, due to hidden blocks, which can waste some of your Star's duration.
There are still some more hidden stuff in the final section of the level. If you have the carrot, you can glide to an alcove to the left of the blocks where the checkpoint bell hangs from, which contains a block that holds a 1-Up. If you don't have a carrot, you can get one by revealing some hidden blocks where the coins are just before the fireplace, which will let you reach the carrot power-up stored in the visible block. The bell is rather simple to reach, just hop up to a higher ledge and jump back down to ring it.
Level 4: Spikes and books[edit]
You've finally reached Rikki's lair, but you still have a little more to go before you can reach his mousehole. You'll need to maneuver around books and spikes while avoiding lots of enemies, but there is no risk of falling to your doom like in Tree Zone. The conveyor belts return, and this time, they'll slowly send you straight into spikes. The 1-Ups in this level are unique in that they will be carried away by a witch when revealed and you need to catch up to her to claim it. What makes it difficult is that the witch can move through solid matter, which can make chasing her a bit troublesome. There is an area above the block containing the carrot power-up that contains a bunch of coins, though you can reveal a hidden block somewhere around it for a money bag worth fifty coins!
Boss: Rikki[edit]
Rikki is a very mobile boss and almost never stops moving, which can make the battle with him quite frantic. He can scale the walls and ceilings and attempt to drop down on you from above, or he can enter and exit any one of the three pipes to try and catch you by surprise. You'll need to time your jumps in order to hit this oversized rat, and three hits will take him out. Occasionally, you can actually defeat him very quickly if you jumped on him while he's on the floor and manage to land on him a few more times as soon as his "getting hit" animation ends, ending the battle before he can move more than an inch.
You have now conquered the Tree Zone, the Space Zone and the Macro Zone. That's three golden coins down, three more to go! While you are still free to choose which zone you'll take on next, but the closest zone is the Pumpkin Zone, which you can reach by going right through a warp pipe.