[@Drakey]

The outside world is dangerous, and not Rat (or Mice) would dwell outside the Mall! BUT we can go with your people living IN the roof, having access to the outside, and farming ground under the cover of glass. 

Thing is, the real Roof is the realm of the Black-Wings and other nightmarish Winged beasts, so an well hidden Outpost, or some scavanging are more realistic!

But Trade will be vital for all factions, and a peaceful or a defensive Clan can/is a viable strategy!