 For the most part the player battles through the game alone, but is occasionally assisted by non-player characters; specifically security guards and scientists who help the player, the former who will fight alongside and both who can assist in reaching new areas and impart relevant plot information. A wide array of enemies populate the game including parasites of Xen such as headcrabs, bullsquids, headcrab zombies and Vortigaunts. The player also faces human opponents, in particular Hazardous Environment Combat Unit (HECU) Marines and black ops assassins who are dispatched to contain the extra-dimensional threats and silence all witnesses.
