I think I know the purpose of the granery (Grainery).

[Dark Redshift]

It's a storage and/or processing centre for demon feed, with the grain taken from DUSK's threshing machine while the rest powers the portal: "Jakob meatloaf with nowt taken out. We make nothing but 100% whole-bonemeal."

https://www.youtube.com/watch?v=2aRj8vyo2S0

And here's the place I first heard about that from, despite being from Britain myself. It's just after the PINGAS scene at 4:09:

https://www.youtube.com/watch?v=cyKGhXv3bXE&t=31s