The Shield of Peace prevents your City from being attacked directly. However, if you leave a portal open on the Kingdom Map, enemies can still attack you through the portal. Moreover, such an attack will ignore your City Walls' defenses and directly engage your army. If the enemy wins the battle, they will plunder your building resources, Food, Silver, and Gold Ingots.
If you lose a fight through your portal, any portals still open on the map will automatically close.