Rally is to attack the enemy with a group of people, your alliance members can join your group. Before the Rally began, the initiator need select a time, after the time, all the admitted members will send thier troops to the enemy.Troops can't to recall after the time. Conditions for using Rally is with building "Headquarters".
Click the base you want to attack>Declare War>choose Rallying time, then some of your alliance members can send reinforcements to your base during Rallying time. When Rallying time ends, both your troops and alliance member's troops will go to fight.
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Game Guide for Rise of Empires