One obstacle course, played solo or together - reach the goal (both of you, in 2-player) to clear it. Touch a spike, get hit by a patrolling spike, fall off, or wait too long on a crumbling platform, and you restart the level. 150 levels, each harder than the last - beat one to unlock the next.
Player 1: Arrow keys to move and jump Player 2 (2-player mode): WASD to move and jump
Hold the jump key longer to jump higher, up to a limit. Orange platforms launch you into the air. Platforms that turn red are about to crumble away.