Perk are unlocked once an Alliance achieves a Prestige Level. So when an alliance unlocks a prestige perk, all the members of that alliance can enjoy the benefits of that perk as long as they are members of that alliance. Once you leave an alliance, all the perk benefits are immediately removed.If you join a new/another alliance, the prestige levels and perks that are unlocked by the new Alliance becomes active.
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Empires & Allies Game GUIDE