The game switches to a third-person perspective during vehicle use for pilots and mounted gun operators; passengers maintain a first-person view.

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.

[13] Halo departs from traditional first-person shooter conventions by not forcing the player to holster their firearm before deploying grenades or melee-range blunt instruments; instead, both attacks can be utilized while a gun is still equipped, supplementing small-arms fire.

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 leader; Jackals, who wear a highly durable energy shield on one arm and a plasma pistol on the other; and Hunters, large, powerful creatures with thick armor plates that cover the majority of their bodies and a large assault cannon that fires explosive rounds of green plasma.

[17] Another enemy is the Sentinels, aerial robots designed by an extinct race called the Forerunners to protect their structures and prevent Flood outbreaks.

[8][13][20] Although the Xbox version of Halo lacks official support for online multiplayer play, third-party packet tunneling software provide unofficial ways around this limitation.

[21] The Windows and Macintosh ports of Halo support online matches involving up to 16 players and include multiplayer maps, not in the original Xbox release.

[9]: 4–5 The game begins as the Pillar of Autumn exits slip-space and its crew discovers a large ringworld structure of unknown origin.

Intent on stopping the Covenant from using Halo, Keyes searches for a potential weapons cache, while Master Chief and Cortana mount an assault on the ringworld's control room.

Cortana enters Halo's computer systems and, after discovering something horrifying, sends Master Chief to find and stop Keyes from continuing his search and uncovering what lies within the ring.

Cortana intervenes before Master Chief can activate the ring; she has discovered the purpose of the installation is to destroy all sentient life in the galaxy, starving the Flood of potential hosts.

Cortana justifies their actions to destroy the Covenant fleet and stop the Flood threat and believes the fight is finished, but Master Chief states they are only getting started.

[28] One of the ideas that the team began to develop was that of a first-person shooter game described by co-founder Jason Jones as "the natural extension of Marathon, which would have turned out to be something along the lines of Quake".

[29] Jones started the design of a 3D engine that could generate height-mapped graphics to visualize elevated surfaces, and he eventually suggested that Bungie use the technology to realize the "tank combat" idea.

[28] A group of three Bungie staffers[31]: 7'02''–7'05''  began to develop an RTS with a focus on science fiction, realistic physics simulations and three-dimensional terrain.

The vantage point continually moved closer to the units as the developers realized it would be more fun for players to drive the vehicles themselves, rather than have the computer do it.

[30] Anticipation built for the unknown Bungie game after favorable reviews from industry journalists under non-disclosure agreements at Electronic Entertainment Expo 1999.

[41] At this point, Bungie promised an open-world game with terrain that reacted and deformed from explosions, persistent environment details such as spent shell casings, and variable weather, none of which made it into the final product.

To make players feel more connected to the action, Jason Jones pushed to turn the game's perspective from third-person to first-person.

To save time, Lehto suggested reusing campaign levels; glowing directional arrows were added after playtesters got lost backtracking.

[34]: 75  Organic, curvilinear forms along with a color palette of greens and purples were used for the Covenant,[30] while the Forerunner came to be defined by their angular constructions; the interiors originally drew on Aztec patterns and the work of Louis Sullivan, before being refined five months from the game's completion.

[citation needed] O'Donnell designed the music so that it "could be dissembled and remixed in such a way that would give [him] multiple, interchangeable loops that could be randomly recombined in order to keep the piece interesting as well as a variable-length".

[30] Ed Fries described the period before the Xbox's launch as chaotic; "You've got to imagine this environment of panic combined with adrenaline, but money's mostly no object at the same time.

Artificial Intelligence were scrapped it became clear that Halo had to serve as the tentpole title for the Xbox,[30] a role which the game was never intended to fill.

[59] After Bungie refused to change the Halo name to appease marketing research teams, the subtitle "Combat Evolved" was added to make it more descriptive and compete better with other military-themed games.

[73] It was released on September 30, 2003,[74] and included support for online multiplayer play and featured sharper graphics, but had optimization issues that caused poor performance.

"[88] AllGame editor Jonathan Licata praised Bungie for doing "a remarkable job with Halo, taking many successful elements from previous standouts in the genre to make one very playable game".

GameSpy commented, "you'll trudge through countless hallways and control rooms that all look exactly the same, fighting identical-looking groups of enemies over and over and over...it is simply frustrating to see a game with such groundbreaking sequences too often degenerate [into] this kind of mindless, repetitive action.

[107] In July 2006, Next-Gen.biz published an article estimating Halo as the second-highest revenue-generating 21st century console video game in the United States, behind Grand Theft Auto: Vice City.

[122] During the Microsoft press conference at the 2011 E3 Expo, it was revealed that Halo: Combat Evolved would be remade by 343 Industries with an in-house game engine and would include achievements, Terminals, and Skulls.