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

Press A to jump and pressed 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.

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

When fighting bosses, bump attack blocks/balls with your head and try to hit the boss with them. Take advantage of barriers like in Space Invaders. Attack blocks/balls take a while to replenish themselves - don't wait, use other ones.

When you have one or two people following you, you'll take damage 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 great 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 them and holding down. This also works on the recurring square yellow object - 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 B 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 in other ways and 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 - I haven't experimented with this, but something tells me he'll want to collect.

If you are being followed by a doppelganger, get everyone, including him, standing in the same place and jump until he's killed.