Citadel Turret is found exclusively in the Derelict Citadel.
A stationary gun turret, similar to the ones encountered in the Ruins and Wastes. However, this model is slightly larger overall and has an enlarged firing chamber. This model fires projectiles that explode into expanding [UNKNOWN REFERENCE] energy circles upon impact. The rate of fire and circle expansion speed is moderately slow, so they should not pose a threat to a trained Scout.
As a whole, the quantity and quality of machine-type hostiles in the Citadel implies that this was the site that the Hivemind was most keen to protect. Despite having achieved a technological level with capacity for interstellar travel, there is no evidence of them escaping the planet. Was there something here that they needed to protect? Or was something preventing them from leaving?
#=/.; that had stood for eons, but she was still too late by a few measly centuries. A city turned sepulchre, it was clear that they had succumbed before they could save themselves. There would be time for eulogies later, now she just had to hope that there was something she could use to;^/(,
(databank entry courtesy of reddit user dratseb)