My walls are destroyed, how to repair them?

When other players attack your city and you are unable to defend it, the durability of your wall decreases and will continue to do so until you complete the repairs. Durability can be repaired every 20 minutes, so make sure to enter the game often to recover the walls as soon as it's possible. If you don't repair the city in time, the durability bar will reach 0 and your city will be relocated to a random location.