This game may seem unremarkable, but it has a few quirks:

Press A to jump and press and hold B to run, of course. Kill enemies by jumping on them (some can't be killed this way). When underwater, press A repeatedly to swim.

You can walk on ceilings by holding up, jumping and making contact with a ceiling. You can walk and jump in this state, but not crouch. Pressing down - on the ceiling or in midair - restores your gravity to normal.

Hitting star blocks with your head makes items fall out of them. Blocks with two-way arrows on them can be pushed.

Falls down pits (or up into the sky) aren't automatically fatal - but they do subtract three hearts from your health.

Go into crouch mode by pressing down, and go out of it by pressing up or jumping. You can walk while crouching.

If a vertically-moving block squashes you against a ceiling, you'll be squashed small enough to walk through one-block-high tunnels. You'll go back to normal as soon as you leave the tunnel. Some vertically-moving blocks will take you to new areas - the first one you'll find, next to a TV that shows a down-arrow when you stand on it, leads to the optional but important area where you can invent the Hell-Pass Device (see below).

When fighting bosses, bump attack blocks/balls with your head and try to time it so you hit the boss with them. During the first boss, take advantage of barriers like in Space Invaders. Attack blocks/balls take a while to replenish themselves - don't wait, use other ones. The second boss is a metronome that only attacks on every eighth tick - when this happens, the two rightmost green apples on the tree you're standing under will spawn red apples that fall, so time your movements.

When you have one or two people following you, you'll take damage if one does, count as falling down a pit if one does, and lose a life if one does. Your controls control them, but on a delay (the second follower will have an even greater delay than the first). This can result in the characters getting separated. If this happens, stop moving while the main character is touching one or both followers - the follower/s will align into the exact same position as the main character for convenience. Making the main character walk against a wall is good for this.

Use phones by standing on them and holding down. This also works on the recurring square yellow object - the Encyclopedia. It will enable you to make an invention. It'll give a cost in gold, but you can make it for less if you're good at rapid button pressing. Press A rapidly to make the invention. Some inventions function with the B button while others are automatic. Many are required to progress. The first invention, the Hell-Pass Device, is optional and special - it doesn't appear in the status bar like the others. Between levels (to end a level, stand on the blocks in the middle of the room of mirrors), you'll visit Hell and get a password from its king. And if you lose all your lives, you can borrow any number of lives from him - but he won't give you passwords any more until you pay your debt.

If you are being followed by a doppelganger, get everyone, including him, standing in the same place and jump until he's killed - then you'll be able to continue.