A city would be a logistical nightmare - and that's just getting enough ammo to kill the ever-respawning zombies. Plus, all it takes is for one or two determined bandits to make hit-and-run attacks on the 'guardians', and their ability to take care of the zombies is shot. Spurdo is right - a barn (or a tiny outpost with a barn and perhaps a few other buildings) would be ideal. For a trading outpost and safehouse, this would work perfectly - guardian players are concentrated enough that it would take a large and well co-ordinated bandit raid to shake them - something that doesn't happen very often.
