Instructions in-game--but here they are if you are a slow reader: A and D or left and arrow keys to move around the circle, and P key to pause the game. Avoid the black things and the mighty laser, collect power-ups, (including special rainbow ones) and rack up points to set the worldwide highscore, or something!
Recently, I found a very niche game called Circumference. It was pretty neat, but I figured it was missing something: namely, power-ups. Everything not related to the power-ups (or the deathbar) belongs to thepterodactyl. Including the music. HIGH SCORE - thepterodactyl with 285!