Heavy artillery emplacement.
The Mega Turret is an automated, long-range artillery platform available to Warlord Pavium. They fire powerful plasma torpedoes that can be upgraded to scorch the impact point and leave a pool of Lingering Death. As if this devastation wasn't enough, they can also use Banished-modified Compound Detonators for even greater damage. These mega turrets are massive and power hungry, and require a standard base building slot.
The Mega Turret was used to protect Covenant military and religious installations, but these massive cannons have also been encountered at remote colonies and outposts guarding nothing of obvious value and reflexively attacking all who approach without long-forgotten authorization codes. The Banished deploy the Mega Turret as a straightforward anti-siege weapon at war camps on the Ark which have the necessary infrastructure and permanence to support their use.