Half-Life is a first-person shooter that requires the player to perform combat tasks and puzzle solving to advance through the game. Compared to most first-person shooters of the time, which relied on cut-scene intermissions to detail their plotlines, Half-Life's story is told mostly by means of scripted sequences, keeping the player in control of the first-person viewpoint. In line with this the player rarely loses the ability to control Gordon, who never speaks and is never actually seen in the game; the player sees "through his eyes" for the entire length of the game. Half-Life has no "levels"; it instead divides the game into chapters, whose titles flash on the screen as the player moves through the game. Progress through the world is continuous, except for short pauses for loading.
The game regularly integrates puzzles, such as navigating a maze of conveyor belts or using nearby boxes to build a small staircase to the next area the player must travel to. Some puzzles involve using the environment to kill an enemy, like turning on a steam valve to spray hot steam at their enemies. There are few "bosses" in the conventional sense, where the player defeats a superior opponent by direct confrontation. Instead, such organisms occasionally define chapters, and the player is generally expected to use the terrain, rather than firepower, to kill the "boss". Late in the game, the player receives a "long jump module" for the HEV suit, which allows the player to increase the horizontal distance and speed of jumps by crouching before jumping. The player must rely on this ability to navigate various platformer-style jumping puzzles in Xen toward the end of the game.
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 guards will fight alongside the player, and both guards and scientists can assist in reaching new areas and impart relevant plot information. A wide array of alien enemies populate the game, including 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.
