From Halopedia, the Halo wiki
Siege Turrets are long-range artillery systems built to counter enemy bombardment platforms that would otherwise be out of range of standard Turrets, and occupy a turret slot. The UNSC learned the dangers of relying on fixed installations against enemies with orbital fire support early in the war against the Covenant. The modular firebase system was developed in response to pinpoint Covenant targeting of large military command and control installations with orbital strikes. and the inability of standard counter battery systems to detect, deflect, or defeat plasma-based artillery bombardment. The siege turrets can do little against threats in the orbital high ground, but they will deter enemy artillery systems by matching or exceeding their range, making ad-hoc assaults on UNSC firebases a costly proposition.
Halo Wars 2
Siege turrets are effective at engaging enemy ground units at range, but cannot fire at units nearby or aircraft. Avery Johnson is able to call in a siege turret anywhere on the battlefield. Ellen Anders' siege turrets are able to spawn swarms of Aggressor Sentinels. Serina's siege turrets are able to freeze enemy ground units. Due to enemies largely ignoring players' firebases in Terminus Firefight, siege turrets are a popular choice in place of standard base turrets. The siege turret's base function is similar to the M400 Kodiak's artillery turret complement, where it cannot attack at close range but it deals high damage to all units except air units.
The cost, however, increased constantly as every additional Siege Tower requested, the player will be charged with additional 150 Supplies and 150 Power. On Campaign, however, the price was fixed to 900 Supplies and 0 Power and the player can requisite it first-time during Hold the Line mission.
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