From what i know the faction trait describes specificly that monsters won't attack units. I don't know how this is with city's and outpost's. I have red somewhere that monsters accidendtly can run over outpost's or city's. In my last game with the Tarth faction i lost 2 outpost's and 1 city's due to monster attacks before turn 120.
Still confusd how this works.
Vivas