By means of this spell, you enchant any normal portal (doorway, archway, window, etc.) to act as a gate for undead monsters. The portal need not be large enough to allow the undead to pass through it, as the magic of the spell causes the undead to appear within range of the portal. Once the undead gate has been placed, it cannot be moved. Furthermore, no more than one undead gate can exist in any given square mile area. Any attempt to create more than one in such an area destroys both gates. An undead gate remains enchanted indefinitely until used once per level of the caster.