In Half-Life, players usually fight alone and only occasionally encounter friendly non-player characters who assist them, such as security guards and scientists. While Decay still features levels where this is the case, significant sections in Decay are dedicated to working with friendly non-player characters, usually escorting them to various objectives and protecting them in firefights.[2] An array of enemy characters from Half-Life populate the game, including alien lifeforms such as headcrabs and Vortigaunts, as well as human soldiers sent in to contain the alien threat. The players have access to a limited selection of Half-Life's weaponry to assist them in the game.[2]