Siege Turrets are long-range artillery systems built to counter enemy bombardment platforms that would otherwise be out of range of standard Turrets. These structures 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.