您在刚刚进入战场时可以获得8小时的免费新手保护。过后您也可以继续购买，城市在攻击别人或遭到攻击时无法使用城市保护。一旦您摧毁或者占领建筑城市保护即会消失。
You can get 8 hours city protection for free when you enter each battlefield. And you can buy it after this free protection. City protection can't be bought while your city is attacking/under attack. It will disappear once you destroy or capture any building.