Diamond Castle is a technique that Adelheid Suzuki uses by using her Glacier Flame to freeze water into a shape of a castle. Adelheid says it is impervious to any physical or natural attack and can repel a flame attack on the level of Tsuna's X Burner. Because of this she calls it her Muro Difesa (Defense Wall) Invincible. However, it's not as powerful on the inside, which leads to it being destroyed by Hibari's Vongola Gear's Needle Sphere Form when he put Roll's porcupine pieces in one of the cracks that he had made by attacking it. He then had Roll multiply to destroy the Diamond Castle from the inside.