In a semi-historical representation of any place or period in history that posits magic and the gods are real, places like the
Panhellenic Sanctuaries could be almost literally impenetrable, not just places that the neighboring city-states agree to leave alone. In D&D terms, the High Priestess of Delphi could be something like an 18th or 20th level Cleric of Apollo, who would have access to spells like
Firestorm and
Divine Word. If she's 18th level, she could cast
Gate and summon a Celestial (or whatever). If she's 20th level, she could literally call Apollo himself to Earth to help (
Divine Intervention, Improved). And that's not even considering the other clerics & paladins, a couple of whom could be 10th or 15th level themselves.