Well rest assured, the chance of that happening is minimum. Other people can only attack you IF you are online as well. And when people are about to attack you, you have to option of waiting for reinforcements, bribe the attacker or fight him.
Eventually a guild will be necessary, since attacking NPC cities is suicide without backup. Plus you really can't experience DoF as a whole if you haven't experienced guildwar related superbattles, where several players from 2 different guilds duke it out on 1 battlefield.
"The flower of friendship blooms even in hell. On the sand where waves come and go, it leaves its petal as a memento. We will make it bloom again someday, The Okama Way!" - Mister 2, Bon Clay