Alpha Base (Installation 04)
Alpha Base was the main tactical firebase used during the Battle of Alpha Halo by surviving UNSC forces after landfall on Installation 04. It was located on a small mesa several kilometers down-spin from the crash site of the UNSC Pillar of Autumn. The mesa itself housed a collection of Forerunner structures as well as a system of caves stretching beneath the facility and linking with the larger subterranean system of the ring.
The humans set up camp around the base using the supplies they had scrounged from the Autumn and their Human Entry Vehicles. They lived in cubicles and ate mostly Meal, Ready-to-Eat ration packs. A good deal of activity also took place in caverns underneath the facility in the Forerunner structures. It was abandoned by the humans shortly before the destruction of the ring, due to their taking of the Truth and Reconciliation and the growing Flood presence in the area.
The Covenant occupied this mesa when they discovered the ring, using mostly a light complement of Unggoy, Kig-Yar, and Sangheili, as well as several Banshees to guard it. It provided a good view of the surrounding areas, and the steep embankments around it made it hard to reach from the ground. Strategically placed Shade turrets also made the mesa difficult to assault from the air.
When the Pillar of Autumn was attacked above the Halo by opposing Covenant forces, the ODSTs, under command of Major Antonio Silva, abandoned the ship aboard Human Entry Vehicles. As the ODSTs fell through the ring's atmosphere, the Artificial Intelligence Wellsley tagged the Covenant base as HS2604, suggesting that it would be ideal as a base for Silva's operations. Wellsley suggested the base be named 'Gawilghur', after a fortress that Arthur Wellesley, later Duke of Wellington, laid siege to in India. Major Silva shot down Wellsley's request and stated that most of the Marines would not be able to pronounce the name correctly.
First attack on Alpha Base
The majority of ODSTs landed around the base. Silva ordered First Lieutenant Melissa McKay to take the butte.  McKay led an assault through a pass that was thought to lead to the top of the butte. As McKay and her company began their climb, the Covenant launched a preemptive strike against Silva. The attack consisted of an estimated one hundred Ghost rapid assault vehicles.
With half of his forces committed elsewhere, Silva ordered his A and D companies to form an "infantry square": an all-round defense formation that, coupled with the hilly natural terrain, gave the ODSTs a better chance of survival against the Ghosts' fixed weapons. To start, the Sangheili drivers fought very poorly. Several Ghosts fired upon each other or collided in their haste to score a kill. Eventually, a more experienced commander took charge and ordered the Ghosts to move around the square counter-clockwise and then to attack the lowest pit in the square. Human casualties increased and the square started to break. Silva responded by reinforcing the pit, ordering his M19 missile operators to rotate their salvoes, and having his sniper team concentrate on the commander. As an apparent afterthought by the Covenant, Banshees attacked late and were quickly eliminated by M19 launchers. Finally, with the commander and 50% of their men dead, the attackers retreated, some carrying extra riders on their Ghosts. From that attack, the UNSC suffered 23 killed in action and 16 wounded in action, of which 6 were critical cases. Finally, Silva was able to order his D77-TC Pelican dropships to attack the butte and give Lieutenant McKay air support and distract the Covenant.
McKay, despite casualties, had made a good deal of progress moving up the butte. At last she made it to a "back door" into the butte. She fired an M19 missile into it, which struck an ammunition store and cleared the room of all hostiles. She and her company entered to find an elevator to the surface of the butte. As a result of the battle, the UNSC managed to gain a base of operations. It is unknown how much of a defense Silva put into guarding this back door, but apparently the Covenant were unable to use it to gain access to the base during later assaults.
Despite now having this strategic foothold, Silva still had a great deal of problems. The ODSTs were equipped with enough food and supplies to last 2 weeks. However, that was not the case with the 63 crewmen that the SPARTAN-II Master Chief Petty Officer John-117 had been able to rescue. Also, the dropships were running low on fuel and the base was not adequately prepared for a Covenant counter-attack. Silva therefore ordered McKay to go to the downed Pillar of Autumn and recover whatever supplies that she could find. The raid was a success with only a minimum of UNSC casualties, while several enemy Ghosts and a Spirit dropship were destroyed. The Marines and ODSTs were able to secure a sufficient amount of fuel, food and ammunition. Two 50mm autocannons were also recovered, a contingent of M12 Warthog Light Reconnaissance Vehicles, and four M808B Scorpion tanks. Unfortunately, no ammunition could be found for the Pelicans, limiting their usage to transport only. Some of the supplies, LRVs, and Marines were lost in the return trip due to Covenant resistance.
Second attack on Alpha Base
- Main article: Battle of Alpha Base
The second battle was Zuka 'Zamamee's attempt to get into the base and kill the Master Chief. Approximately six Spirit dropships full of Covenant and the captured Pelican Charlie 217 full of cloaked Sangheili commandos participated in the attack. The Pelican managed to touch down and release the Sangheili within before its landing pad was doused in fuel and lit. However, Zuka 'Zamamee and several Sangheili had already fled and began in search of the Chief. Before being killed by the Stealth Sangheili led by Zamamee, a valiant crewman, Cho, gave the AI Wellsley control of the 50mm autocannons, with which he was able to eliminate 1 Spirit dropship. The remaining dropships released their troops further away from the base and attempted to assault it thus. Snipers expertly inflicted heavy casualties on the disembarking troops. Two of the recovered Scorpion Tanks had been entrenched and were able to cut down much of the onslaught, including the Field Commander. With their leader dead, lesser Sangheili attempted to assume command with little success. In addition, the Covenant were able to employ the Type-56 Lich Upright Chalice in their attack, using it to deploy vast quantities of troops onto the base directly, before ODST Fireteam Raven were able to hold off the attack.In the end, all remaining Covenant were either captured, killed, or forced to retreat. Yayap and Zamamee were able to survive by stealing one of the Banshees that the Marines had been able to salvage.
It was later discovered that the Flood had been attempting to break into the base and were only impeded by a single metal grate (which they had already used hundreds of plasma weapons on), leading to an unnoticed group of caverns. The entrances to these caverns were successfully sealed, and the Flood were delayed from overtaking the base. By the time they managed to break through, the UNSC forces had abandoned the base in hopes of leaving the installation aboard the Truth and Reconciliation.
The base was destroyed, along with Installation 04, when SPARTAN-117 overloaded the Pillar of Autumn's reactor.
The defenses of Alpha Base consisted mainly of UNSC personal that survived the crash of the UNSC Pillar of Autumn. Two stationary 50mm autocannons were in place and linked to the base's AI, Wellsley. The base had several makeshift landing pads where dropships could be docked and refueled in relative safety. Later, this position was reinforced by a group of LRV Warthogs and at least two of four Scorpion tanks that were recovered from the downed Pillar of Autumn.
Due to successful raids and the attack upon the Truth and Reconciliation, at least four Shades, were set up in defense of the base. Unfortunately, these turrets had limited effectiveness against dropships. Also, 15 operational Covenant Banshees were recovered. A basic perimeter was set up and monitored through motion trackers by Wellsley. Pelicans had quick strike teams in place if there was a perimeter breach that could respond in about two minutes. Finally, a combination of LRVs and captured Ghosts were used as patrols, effectively combining power and speed.
At its strongest point, the bases defenses were as followed:
- Battalion of ODSTs led by Major Silva.
- Most of the Pillar of Autumn's remaining crew that were not captured or in cryo stasis.
- Two 50mm autoturrets (AI operated).
- High amounts of light and heavy weaponry (both UNSC and Covenant grade).
- Four Shade turrets.
- At least two of four remaining Scorpion Tanks, dug partially underground for protection.
- A contingent of LRV Warthogs.
- 15 operational captured Banshees.
- Captured Ghosts (estimated at about 5-10).
- AI Wellsley.
- Multiple landing pads.
- All surviving Pelicans, but these were low on machine-gun ammo and had no rockets.
- Ground based motion detectors.
- A system of trenches surrounding the base.
- One of the four Scorpion tanks recovered from the Autumn is presumed to be the one encountered on Assault on the Control Room.
- The appearance of Alpha Base, as depicted in Halo: Fireteam Raven, bears extremely heavy resemblance to the Forerunner structure found in the skybox of the Halo: Reach Anniversary Map Pack map Ridgeline. Notably, the doorway structures leading to the base's interior sections bear heavy resemblance to the map's red and blue bases and the overall structure exhibits extremely similar architecture. furthermore, Ridgeline's map description states that "This idyllic cliffside once served as a UNSC staging area", indicating that the staging area referred to in the map description is, in fact, Alpha Base (or a War Games map based on Alpha Base).