Sonic the Hedgehog is a 2D side-scrolling platform game. The gameplay centers on Sonic the Hedgehog's ability to run at high speed through levels that include springs, slopes, bottomless pits, and vertical loops.[3] The levels are populated with robot enemies, inside which Dr. Robotnik has trapped animals; destroying a robot frees the animal, but is not necessary to complete the game.[4] The player must also avoid touching spikes, falling into bottomless pits, being crushed by moving walls or platforms, and drowning, which may be prevented by breathing air bubbles from vents.[5] Sonic's main means of attack is the spin attack, in which he curls into a ball and spins his body, damaging enemies and certain obstacles upon collision. This may be performed by jumping or rolling.[6]