So aside from the basic outline what do you have? What's the city like? is it a agriculture city or a hive or a century old city?
all I can think of right now.
Given what I was thinking of regarding Bargose, I'm thinking it's a besieged Forge World, one of the smaller more minor capacity style forge worlds, heavily industrialized, but not a forge world setup for the likes of Titans or Super heavies but something like Leman Russes or Chimeras. So it's a supply node for the Imperial Guards, but not the Space marines so the Imperial Fists would respond to the distress calls of the Guard and the.... the dudes with machines that my brain is refusing to think of the name for but yeah. The imperial respond because they have to, not because their obligated to, so they're deploying select forces to support and aide allied forces rather then an out right detachment to save the world itself.