Improved Elemental Protection (Stronghold Augmentation)

Elemental Protection: This magic augmentation causes the wall to absorb the first 120 points of damage from a specific energy type (acid, cold, electricity, fire, or sonic) per day. If all 60 points of protection are expended, they return 24 hours later, if the wall is still there.
Caster Level: 10th; Prerequisites: Craft Wondrous Item, protection from elements; Market Price: 15,000 gp (improved elemental protection).