What does Mickey do besides walk?
The bounce attack and vine immunity are the two mechanics worth practicing before the first boss.
The bounce attack is everything
Mickey's main damage tool is the bounce: press jump once, then press Space again in the air and keep it held through the stomp. If you let go before he lands on the enemy, the enemy hits you instead of dying. Chain bounces between enemies to cross gaps without touching the ground, and hold jump on contact to launch higher than a normal jump can reach. That second trick is what gets you onto the upper platforms in Toy Land and through the gear stage in the Castle. The same input also slides Mickey safely down slopes. Indie Gamer Chick credits this game as the platformer that invented the manual butt-stomp, the jump attack that requires a deliberate second press rather than coasting on gravity.
Vines make Mickey invincible
While Mickey is hanging on a vine, nothing can hit him. Butterflies that fly into the swing arc die on contact instead. The straightforward use is survival: in the vine section of Forest stage 1, grab a vine and ride across rather than trying to bat butterflies away between jumps. The fun use is point farming. Stay on the vine, swing back and forth, and the butterflies feed themselves into you for as long as you want to sit there.
How do you actually beat the bosses?
Mickey's projectile inventory caps at 30 items and resets every time you walk into a new stage. Don't enter a boss with apples saved for "later". There is no later. Empty the bag.
Faceless Tree (Forest)
The face rolls out of the stump and chases Mickey across the screen. After each hit it dumps roughly five acorns from random positions overhead. Apples take it down in about ten hits and let you keep your distance. Bouncing cuts the count to five or six, but you're timing your landings between falling acorns: faster path, harder run, and you'll eat at least one acorn most attempts. If you brought a full bag of apples through Forest stage 2, this is what they're for.
Jack-in-the-Box Man (Toy Land)
He bounces a couple of times, then fires four corkscrew springs into the air. The springs are your platforms. Jump from one up to his head and bounce. After your second hit a boxing glove punches out at chest height; duck or run under it. The fight loops cleanly, so the first cycle is the tutorial.
Storm boss
A fish leaps out of the water, arcs across the screen, and punches at ground level when it lands. Ducking won't save you here, the way it does against the Jack-in-the-Box glove. Hit it at the peak of the leap, while it's hovering for a frame in the air. Each clean stomp drops one of the five statue segments behind it. Five hits, same pattern every cycle.
Licorice Dragon (Library)
The dragon pops out of one of four holes in a fixed order: far left, center-left, far right, far left, center-right, repeat. Once you've memorized that, the fight stops being a fight. Stand on the platform next to the next spawn, jump as the head emerges, bounce. The pattern resets after the fifth pop.
Oafish Clock Maker (Castle)
The Clock Maker blocks almost everything you throw at him until he grins. When he bares his teeth, he's open for one beat. Jump, tap Space as you come down on his head, slide off to the far side, wait. His face flushes redder with each clean hit. Eight hits to finish him. Slower than the others, but cleaner once you read the tell.
Mizrabel
Every other boss in the game punishes contact with their projectiles. Mizrabel is the exception, and the exception is the entire fight. You can stand on her missiles. Five hits and she falls.
Her pattern: buzz around the chamber, land at one of four corners in a diamond, pulse her shield in and out three times, then fire missiles in a hexagon spread. Camp one of the small upper ledges. Wait for the launch, jump toward her, hold Space on the way down to bounce off her head. If she lands on a side rather than a corner, jump on her shield while it pulses; when the missiles fire, drop onto her from above and stomp.
Skip Practice mode
The mode menu lists Practice as the easy option. It's a trap. Practice gives you truncated versions of the first three levels and removes every boss, including Mizrabel, so the credits never roll. If you're playing Castle of Illusion online for the first time, pick Normal. Hard mode also reaches the ending, just with sharper enemies. Practice is good for one thing: learning the platforming basics. Once you've cleared a stage there, switch.
Free lives and hidden rooms
How to earn a free try on every level
When the end-of-level bonus screen comes up and the points start tallying, hammer Enter until you hear a small chime. That's a free try added to your count. The window stays open for the entire bonus tally. Works on every level, including the ones leading into bosses, which is exactly when you want the cushion.
Three hidden rooms worth a detour
Forest stage 2: after bouncing off the line of ghosts, drop into the hole below the branches, stomp the mushrooms, then keep walking left into what looks like solid wall. The wall isn't there. Inside is a small room packed with diamonds.
Dark Canyon stage 1: head left at the very start instead of right. Same invisible-door trick as the Forest detour, easy to miss because the level is begging you to push forward.
Toy Land stage 2: in the spring section, jump on a spring and hold hard right at the apex. Mickey lands in a compartment built into the wall. Smash the bricks with a marble for a Star.
Toy Land's hidden screen-clear
One item in the entire game wipes everything off the screen at once: the Up/Down Arrow, and it only appears in Toy Land. Pick it up and the camera flips upside-down for a few seconds while every enemy on screen dies in place. The flip is the visual gag, but the kill is the point. The arrow can be re-triggered by retreating off-screen and stepping back in. That makes it especially useful when clowns and airplanes start sharing the screen.