Rune of Impregnable Defense (Cooperative)

This spell is used to strengthen the fortifications of a dwarven stronghold by increasing the resistance of a castle wall, gate, portcullis, drawbridge, or other defensive structure against physical or magical attack. The spell is cast by a circle of dwarven clerics linked by a combine spell. Next, a large rune is inscribed on the surface of the structure to be protected in the last turn of the spellcasting. The rune is created to protect a surface of up to 2,500 square feet in area (such as a 50-foot by 50-foot section of castle wall). The rune is invisible after the spell is cast, but can be revealed by a detect magic spell.

The protection provided by the rune is not permanent: it expires after one month, and the rune fades away if the spell is not renewed. To renew the rune's power, a dwarven cleric of at least 7th level experience must cast a glyph of warding spell upon the rune before it fades; the rune is thus renewed and remains in effect for another month. The dwarves must constantly renew the runes protecting a stronghold every month on a regular schedule, or else the runes fade away and become useless.

This powerful rune protects a surface from the attacks of battering rams, siege machines, stones hurled by catapults or giants, ballistae missiles, or other such physical attacks that inflict structural damage. All physical attacks inflict no damage to the surface. A section of castle wall or stone tower protected by this rune cannot be undermined and collapsed by sappers, because the rune maintains the strength and integrity of the structure and its underlying foundation.

This rune also provides protection from many magical attacks. An attack that inflicts structural damage, such as a lightning holt, disintegrate spell, or a horn of blasting inflicts only half damage if a successful saving throw vs. spell is made. The saving throw made by the structure is that used by a 19th level or greater cleric. Thus, if an earthquake spell strikes a structure protected by this rune, the earthquake's damage is reduced by 50% if a saving throw of 7 or better is rolled. Also, if a saving throw vs. spell is made against a passwall spell, the passwall does not take effect.

This rune has one additional protective function. If a prayer spell is cast upon the rune by a dwarven cleric, any structural damage suffered by the protected structure repairs itself at the rate of two points of damage per round. The repair continues for as long as the prayer spell is in effect.

It is possible for the rune to be destroyed or dispelled, of course. If the surface upon which the rune is inscribed is utterly destroyed, the rune is also destroyed, and no repair can take place if a prayer spell is cast upon the rune. A section of castle wall reduced to rubble or a drawbridge smashed apart also loses its protective rune. A dispel magic spell cast by a high level spellcaster might possibly dispel the rune. Again, the rune is treated as if cast by a 19th level or greater cleric.

The material components for this spell are a golden writing stylus with a diamond point (worth at least 5,000 gp), which is used to