Main Worlds
This is the main world, and it serves as a hub area. All other levels are reached via this hub area. As you collect Stars and Keys, more areas become available and more worlds are opened. There are numerous Power Stars scattered around the castle itself, and the castle now has an infestation of rabbits who hold keys to unlock the various mini-games.
The first of the painting worlds. As the name suggests this is home to the explosive Bob-Ombs, both the black enemies and the pink allies. This world introduces the player to many game aspects, including the cannons, flying and boss battles. Koopa the quick waits for a speedy challenge and a Chain Chomp guards a valuable prize. Watch your step though. Areas of the stage are subject to bombings.
The first stage where it's possible to fall off the stage and instantly lose a life. Whomps are large walking pavement slabs. Although big they are also somewhat clumsy, which often results in exposing their vunerable backs. Thwomps, the more blocky and more resistant versions, also appear here. An owl offers a lift into the sky and a tower sits in the centre, waiting for adventurers to scale it.
Jolly Roger Bay
Your first taste of deep water, you have to be careful to watch your air supply while making your way through the watery depths here. A giant eel hinders your progress at several points. Plundering a sunken ship and raiding a cave reached via an underwater path are some of the activities to complete here.
Cool, Cool Mountain
Yoshi and friends feel the chills in this snow-covered level. Snowman makes their appearance, which requires an alternative method to defeat than most. An icy slide hides several prizes, and the penguins play important roles in this stage. Watch your step though, there's nothing to stop your sliding off the mountain into the abyss.
Big Boo's Haunt
A stage that takes place in and around a large spooky mansion. The ghosts tend to be shy of visitors, but take care if you turn your back on them. Mysterious happenings like flying books and moving furniture are commonplace here. There's even rumours of a famaliar plumber's voice echoing in the rooms...
Hazy Maze Cave
Be careful when exploring these caves that you do not get lost. The many passageways and caverns that make up this stage can be quite confusing. The cave-dwelling creatures don't take kindly to outsiders either. Make sure to stop by the underground lake and hold your noise in the hazy maze itself.
Lethal Lava Land
A stage with a hot surprise to those careless enough to slip off the platforms. Even worse, there are some bullies here determined to make you experience that. Well, give as much as you get, and punch them into the boiling lava. There's plenty of lava to negotiate outside, and more to avoid inside the volcano too. Watch out for flamethrowers and sinking/tilting platforms too.
Shifting Sand Land
A stage mostly split into two parts. The first is the hot desert environment. Quicksand is scattered throughout, making a swift demise an ongoing risk. A condor roams the skies, and swoops down on unsuspecting victims. One part involves making your way around concrete walkways, with moving blocks threatening to crush you and quicksand around you. The second part takes place inside the pyramid. The risk of quicksand has been reduced, but now there are more enemies to contend with, as well as several bandaged Thwomp enemies to contend with.
Dire, Dire Docks
The second of the water stages, this level is split into two main chambers. The first chamber has numerous underwater creatures, treasure chests and a vortex that threatens to swallow anything that strays too close. The second area involves pole jumping and use of special abilities to bypass obstacles. Bowser even has his submarine docked here at one point.
The second snow level. This stage has many more snowmen than before, as well as numerous new hazards to worry about. Some enemies disguise themselves as coins until you get near, and the water here is hazardous to your health. The large snowman doesn't take kindly to anyone trying to cross the bridge and treasures await those who venture into the igloo.
Wet Dry World
The third of the water stages. This stage's unique trait is the water level. By using the diamonds to alter the water level different areas are opened and closed. The water level is even affected by how you enter the level. The various areas of the stage are very dependant on the water level, and there's even an entire downtown section only reachable when the water level is at its highest.
Tall, Tall Mountain
A mountain with many tricks and hazards on the way to the summit. Wind gusts threaten to blow you into the abyss, monkeys cause various problems and mushroom hopping can prove hazardous with a misjudged jump. The mountain also houses various secrets, such as warp points and a slide.
Tiny Huge Island
A stage with two sides to it; tiny and huge versions of the same stage. Green warp pipes are used to switch between the two versions. What are little threats like tiny Goombas and Pirahna Plants become towering terrors. A short sprint transforms into a lengthy run, and previously easy jumps become out of reach. The mountain houses valuable treasures, but beware of the protective measures in place to stop you. There's even a racing rematch somewhere round here.
Tick Tock Clock
This entire stage takes place inside a massive clock. Many elements of the stages are usually moving, such as blocks, cogs and the clock hands. Enemies lurk, waiting to try and send you falling into the nothingness below. The moving hazards present the same hazard. How you enter the stage determines how these objects move, with the ability to stop them completely if you enter at a certain time. Some Stars cannot be retrieved when the clock is stopped though.
The final main world takes place high in the sky, with rainbows streaming all around the place. The risk of losing a life to falling is at its highest here as you make your way along the platforms. Ride the carpets, explore the red coin structure, visit the airbourne house and step aboard the flying ship in your search of those Power Stars.
Credit goes to Insanity Prevails
 Mini Worlds
The Princess's Secret Slide
Reached via one of the windows in the Princess's room. It's a simple twisty slide. A blue coin block sits in the corner, and two stars lie as rewards, woth one harder to get than the other. Falling off the slide simply returns the player to the main hall in the castle.
Reached via the unlocked double door in the Princess's room. The goal of this small tropical stage is to collect the 5 silver stars to gain a true power star. There are a few enemies to worry about to make things interesting.
The Secret Aquarium
Reached via one of the wall holes in the room with the Jolly Roger Bay painting. The objective is simply to collect all eight red coins. However, with no air pockets your only source of air are the coins themselves.
Reached via the 8 star door in the mini-game room. You need to clear this stage to unlock Mario. There are numerous platforms scattered around, with poisoned floors or, even worse, a fall into death for the careless. An oversized Goomba is the final challenge, and 8 red coins await a return visitor.
After collecting 12 stars a light begins to shine into the main hall. Looking into the light transports you to this stage. Only Mario can succeed in this stage, as only he benefits from the wing cap. The switch lies on the central tower, and eight red coins are around the skies here.
Bowser in the Dark World
The first of the Bowser stages. Falling off the stage results in instant death, so watch your step. This stage is filled with hazards, including flamethrowers, spark balls and Goombas. The eight red coins scattered around is even more difficult, requiring excellent acrobatic skills.
The Secret of Battle Fort
A stage reached via the hole in the castle courtyard, although Yoshi can't enter due to the lack of strength needed to smash the blocks away. There are numerous hazards, including Goombas, rolling black balls and enemies that intend to throw you about. The goal is to collect the 5 silver stars to earn a power star.
Big Boo Battle
Reached via the Luigi painting in the attic area of Big Boo's Haunt. This stage is comprised of a series of rooms, and a maze of doors. Take the wrong door and find yourself back at the beginning. At the end of this waits Big Boo as the boss fight, with an interesting use of a mirror for battle. This stage must be cleared to unlock Luigi.
Behind the Waterfall
Reached via the Underground Lake of Hazy Maze Cave. Some of the red coins here require the use of the metal ability to retrieve, meaning tackling this stage without Wario is quite pointless. If you get caught in the rapid water you'll be swept to outside the castle!
The Secret Under The Moat
Reached by draining the moat and then smashing the black block away (requiring Wario). The first part involves a slide down, and then negotiating a series of platforms and tilting ledges. There are two stars for collection in this stage.
Bowser in the Fire Sea
The second Bowser stage. Unlike the previous one falling from the stage only results in a dip in the surrounding lava, meaning a chance of surviving. Difficulty of negotiating platforms, as well as actual hazards, has been taken up a few notches. Bowser even has a new trick, which involves tilting the entire platform you're on.
Chief Chilly Challenge
Reached via the Wario painting in the mirror room using Luigi's ability. This stage is comprised of various icy platforms and slides. Be careful not to slide off the edge. Bullies and strong winds hinder progress throughout. The ultimate bully awaits the one who makes it to the end.
Over the Rainbows
Reached via one of the wall holes in the room with the large clock. This stage takes place high in the sky. You must make use of the cannons and Mario's wing cap ability to fly around. Falling from here won't cost a life but instead will take you to the lake outside the castle.
Bowser in the Skies
The final Bowser stage. Unlike past Bowser stages only Mario can tackle this one even after clearing it once. As with the first Bowser stage death awaits those careless enough to fall from the stage. This stage is filled with hazards, enemies and tricky platforms to negotiate. The Bowser awaiting Mario this time has better endurance and more tricks, like causing parts of the platform to disappear.
Credit goes to Insanity Prevails