Multiplayer & Interactions with other players

• How many times per day can I plunder another player's city?
• How do I visit cities of other players?
• Can I change my neighborhood? I'm being constantly plundered.
• How can I protect myself from being plundered?
• I was plundered despite having a city shield! How is this possible?
• How does the friend list work? What are the limits?
• What is the player ranking?
• How can I motivate and polish buildings of other players?
• What is the social bar and how does it work?