Diamond Castle is an ability used by Adelheid Suzuki through her Glacier Flames. She refers to it as a Muro Difesa Invincibile (
Description[]
Adelheid uses it to freeze water into a shape of a castle. It is impervious to any form of attack, physical or flame-enhanced. It can also repel a flame attack on the level of Tsuna's X-Burner.
However, it is not as powerful on the inside, which leads to it being destroyed by Roll's porcupine pieces in one of the cracks that Kyoya Hibari had made by attacking it.