The Five Peaks can be attacked only by flyers or earthwalkers.
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TSR
Wizards of the Coast
It is a great choice to use this Birthright realm since it drastically limits the attackers. This realm can only be attacked by champions and allies who are able to earthwalk or fly. The Five Peaks are simply out of reach for all other champions. When defending against these types of champions/allies, it is important to use cards that work well against them. A few examples of cards that would be useful in defending the Five Peaks would be Noble Djinni (84/3rd), Gloriana (3/1st-chase) and Wind Dancers (275/3rd). Due to this restriction, this realm is a good choice for your front ranks, since so few champions will be able to attack.
It is not recommended to play this card yourself when you have the Five Peaks in your formation, since it is subject to the powers of Volcanic Eruption (64/RR). Hold off on playing Volcanic Eruption if you know your opponent has it in his or her deck, or play Calm (400/3rd).