Having a settlement you worked hard to build be under constant attack just isn't fun. Pathfinder Online
realizes this, which is why settlements are normally defended by large numbers of respawning NPC guards. Not too many to prevent players from scouting a town or stealing a few things, but enough to make a frontal assault fruitless unless you want your corpses to litter the outskirts of town.
If that were always
the case, though, attacking a settlement would be pointless. So during a brief window every day, the guard respawn rate decreases, numbers thin, and suddenly the town looks much more vulnerable to attack. A well-developed settlement will still have guards, but a coordinated force will be able to eliminate them... unless players step in to defend the settlement. This also solves the problem of having massive attacks come when no one is awake to know what's happening. Read the full details in the latest development blog on the official site