Atlantis' shield

Atlantis has a shield that when span across the city is shaped like a giant bubble encupesing the city.The shield protects atlantis and the things inside it from vurtaly anything.Unlike a space ship where the shield depletes but can then recharge when the shield on atlantis depletes the zpm(s) deplete in power.The shield can protect atlantis and any thing in side from any thing until the power depletes.