Shields are also called bubbles. There are available in three time lengths, eight hours, 24 hours, and three days (72 hours).
Shields protect your city from attack by other players. They do not protect your troops who may be out of the city, farming a resource plot or camping.
While your shield is in place, you can attack zombies in the surrounding territory. You cannot scout or attack another player without dropping your shield.
A shield is useless in preventing zombie attacks during the Zombie Siege activity.