Lightning Spires are guardians that are quite similar to the Gargoyle Traps and the Fire Towers. Most of them are located within the Arcane Sanctuary. However, instead of shooting Fire Balls, these sentries shoot Lightning bolts and use a skill similar to Nova.
Like Gargoyle Traps, they are extremely hardy and can take a large amount of damage. Many adventurers choose to avoid the lengthy process of destroying them as they do not drop items. Life and Mana also cannot be stolen from them.
On Normal difficulty, Lightning Spires are immune to Poison, and on Nightmare and Hell they are immune to both Poison and Lightning.
Background
Left behind by paranoid sorcerers to safeguard their sanctuaries from thieves, these enchanted spires emit bolts of lightning at those who approach too closely. An interesting quirk of these guardians is that they are not activated by the proximity of the many monsters that roam the areas where they are found. This has led many sages to speculate that they may have been created even before mankind knew of the existence of Demons.