From what I've experienced, having players protect other players is not going to work, most people are just going to find ways around it like in real life or actually make things worse for the people they're protecting, there is no real integrity if you have enough people and corruption is always present. Your best bet is having save zones enforced by the game mechanics, but what I'm suggesting is giving players a way to protect themselves against people abusing systems without relying on other players, because trust in other people is not something you can rely on to keep things fair in a game, while theoretically allowing player owned cities, while keeping them in line with basic expectations.