Build the level by placing blocks, projectiles, traps, and hazards, then try to reach the flag. If you can make it but your friends can’t, you get points! Repeat the process as you place more platforms to make the level crazier and more dangerous, and try to screw your friends over without screwing yourself. The result of this unique game flow is that every player becomes a designer and a player at once, responsible for creating dastardly levels without ever feeling like they're really going into "level editor" mode.
You score points for reaching the end of the level (as long as at least one other person fails), for getting there first, for placing traps that stopped other players, for being the only one to reach the end, or for picking up coins placed by players and bringing them to the end of the level.