The Shield of Peace grants you protection from enemy attacks. When you start playing for the first time, you will be provided with a Shield of Peace for 7 days and other players will not be able to attack you, allowing you to grow and develop.
You always have options to purchase a Shield of Peace either in your Warehouse's "Battles" section or with packages that are available on your offers screen. You do not have to use a purchased Shield straight away, i.e. you can keep it for later after purchase.
The time of several Shields doesn't sum up.
Note that you can still be attacked while under the Shield of Peace protection.
If you have a portal open anywhere on the Kingdom map, enemy players will be able to launch a march towards your portal. In this case, once their troops reach your portal, there will be a fight, while the enemy will ignore your City Walls and the Shield of Peace.