How can I protect myself from being plundered?
The best way to protect yourself from being plundered is to always be ready to collect your productions when they finish. A little bit of planning goes a long way, and plundering won't be very fun if there's nothing to plunder!
Aside from that, having a strong city defense is often also a good way to protect your city. By using the blue "shield" button in your army management you can designate units to your city defense.
Having a variety of units of your current age as well as some special units like heroes can help a lot. Remember that your city defense can't die permanently (although they can be defeated), so using a special unit in it won't put it as risk of being killed!
Units that are part of your city defense have a blue outline in your army management:
Simply tap one of the units and then select the blue "shield"-icon to add it to your city defense:
Lastly, the Friends Tavern also offers a city shield for a bit of tavern silver. This will prevent other players from mounting an attack against your city and thus also prevent them from plundering it. Keep in mind however that it won't prevent plundering in general, so it cannot be used to block the player when the attack has already been successful!