Zone of Elemental Immunity

The stronghold space is immune to damage from a specific energy type (acid, cold, electricity, fire, or sonic). Anyone within the stronghold space shares this immunity, but only if the damage comes from a source outside the stronghold. For instance, while the stronghold floats on a lake of lava, those within suffer no damage, but a fireball cast from within the space affects all creatures within normally.

Unless you have some specific reason to protect only a section of your stronghold, this effect is generally placed on the entire stronghold.

Caster Level: 5th; Prerequisites: Craft Wondrous Item, protection from elements; Market Price: 10,000 gp.