Halo: Combat Evolved is a shooter game in which players experience gameplay in a 3D environment almost entirely from a first-person view (FPS). The player can move around and look up, down, left, or right.[6] The game features vehicles, ranging from armored 4×4s and tanks to alien hovercraft and aircraft, many of which can be controlled by the player. The game switches to a third-person perspective during vehicle use for pilots and mounted gun operators; passengers maintain a first-person view.[7] The game's heads-up display includes a "motion tracker" that registers moving allies, moving or firing enemies, and vehicles, in a certain radius of the player.[8]
The player character is equipped with an energy shield that nullifies damage from weapons fire and forceful impacts. The shield's charge appears as a blue bar in the corner of the game's heads-up display, and it automatically recharges if no damage is sustained for a brief period.[8] When the shield is fully depleted, the player becomes highly vulnerable, and further damage reduces the hit points of their health meter.[9] When this health meter reaches zero, the character dies and the game reloads from a saved checkpoint. Health can be replenished through the collection of health packs scattered around the game's levels.[8]
Halo's arsenal consists of weapons from science fiction. The game has been praised for giving each weapon a unique purpose, thus making each useful in different scenarios.[10] For example, plasma weapons need time to cool if fired too rapidly, but cannot be reloaded and must be discarded upon depletion of their batteries, whereas conventional firearms cannot overheat, but require reloading and ammunition. In contrast to the large weapon inventories of contemporary FPS games, Halo players may carry only two weapons at once, calling for strategy when managing firearms.[11]
Halo departs from traditional FPS conventions by not forcing the player character to holster its firearm before deploying grenades or melee-range blunt instruments; instead, both attacks can be utilized while a gun is still equipped, supplanting or supplementing small-arms fire.[8] Like the game's other weapons, the two types of grenades differ; the fragmentation grenade bounces and detonates quickly, whereas the plasma grenade adheres to targets before exploding.[12][13]
The game's main enemy force is the Covenant, a group of alien species allied by belief in a common religion. Their forces include Elites, fierce warriors protected by recharging energy shields similar to the player's own; Grunts, which are short, cowardly creatures who are usually led by Elites in battle, and often flee in terror instead of fighting in the absence of a leading Elite; Jackals, originally space pirates, who wear a highly durable energy shield on one arm and a form of handgun on the other; and Hunters, large, powerful creatures composed of small worm-like colonies with thick armor plates that cover the majority of their bodies and a large assault cannon that fires explosive rounds of green plasma.[14] A secondary enemy is the Flood, a parasitic alien life form that appears in several variants.[15] Other enemies include Sentinels, aerial robots designed by an extinct race called the Forerunners to protect their structures and prevent Flood outbreaks. Sentinels are able to hover around in enclosed spaces and produce an energy shield when under attack. They lack durability, but use powerful laser weapons and are immune to infection by the Flood.[15]
The artificial intelligence in Halo has been favorably received.[16] The player is often aided by United Nations Space Command (UNSC) Marines, who offer ground support, such as manning gun turrets or riding shotgun while the player is driving a vehicle.
