User blog comment:Zathronas/Speculative author: Where are the Grimm?/@comment-23790511-20140914035557/@comment-24891101-20140914045258

Varzan: I've always thought of the Grimm as not actually being alive in any conventional sense, for precisely the reasons you mentioned. I'd also like to not go the "poor, misunderstood, provoked beast" route. Either an emergent property of a universe which dislikes souls, or a weapons system gone awry, created by a vast and incomprehensible, but long passed intelligence. As such, they have no motivations, merely seeking to snuff souls, both hungry for and despising them. Drawn to souls like moths to a flame, like lightning to earth, they reproduce in the dark places, where there is little in the way of life, where the ambient background glow of souls is low. Humans have reached sufficient population and technology that they have been beaten back, and the regions immediately outside the cities are reasonably safe, while within a Grimm attack is a very rare occurrence.

In general, walled citadels makes sense. The opening map had fortresses, Vale's city limits look suspiciously like walls, air and sea travel are the primary transportation, what freight rail they have is elevated and heavily guarded, the kingdoms have been historically isolated, and the schools maintain the motif of islands of civilization beset on all sides by extreme danger.