When you are dealing with a powerful enemy that is difficult to win by yourself, you can ask your Alliance Members to rally troops with you for a more powerful attack called a Rally Attack. To do this, you can tap the enemy's Castle and select the option for a Rally Attack. This will give you the option to choose your desired rally wait time and will also display the troops sent by your Alliance members as they gather in your Castle. At the end of the waiting period, the troops that have gathered will begin marching to the target, and the buffs and bonuses of all allied attacking units will apply to the Rally Attack.
Keep in mind that a Hall of War must be active to use the Rally Attack feature.