Generator Defense could only be exclusively played on the map Overlook in Halo: Reach Multiplayer Beta. Overall, the gametype is basically an attack-and-defend type of gameplay. There are only two rounds in the gametype; a total of five minutes in each round.
The defending team consists of three Spartans that defend a set of three generators, while the attacking team consists of three Elites that have to fight their way up to the Generators. There are a total of three defense Generators on the map which the Spartans have to defend. At the beginning of each match, several weapon caches will be distributed by a Longsword in different locations on the map; each weapon cache can potentially be a Spartan Laser, a Sniper Rifle, a Target Locator, or a Rocket Launcher. Each weapon cache will be dropped evenly around the map to allow both teams an equal chance of acquiring them. The defending Spartans will be able to lock the Generators down—hence, becoming invulnerable to attacks—by pressing the action button near one. The lockdown lasts for 30 seconds and will start a cooldown time after the active time period.
Generator Defense is a playable gametype in Firefight, and on the defending team, instead of fighting player controlled Elites, Spartans will fight waves of the Covenant. Players will also be able to customize the amount of Generators on the map.
- Network Test 1 was actually a test of Firefight capabilities in a co-op setting without letting players actually play these gametypes. This is shown by the fact that Screenshots and Films are described as "Overlook on Easy" or "Overlook on Heroic" as opposed to the usual "Network Test 1 on Overlook". Also, the game is affected by lag much the way a campaign game is strained, not to mention the fact that Overlook appears perfect for Firefight, complete with weapon drops and ammunition.
- As it was only a Network Test, during the Halo: Reach Multiplayer Beta, players often confirmed that gameplay was sufficiently 'laggy' and became harder to play online. Bungie fixed this problem when the full game was shipped.
- Due to the fact that Network Test was in its early stages of development during Beta, there game mode had a tendency to fail to correctly read the codes for the Spartan models as the player models were usually colored "black and white" or "pink and brown" instead of "Spartan green". When viewed in first-person, the customized Spartan colors are displayed with their correct personalized colors. Shoulder permutations also had a tendency to vanish and/or fail to load their appropriate textures which resulted in the shoulder permutations being white/silver silhouettes.