Carroristo is an iron fortress on the Abyssal layer of the same name (numbered the 499th by the Fraternity of Order). It's located on a rocky island in the middle of an acidic sea protected by aquatic Tanar'ri.
The fortress of Carroristo was the site of a historic battle where the Baatezu attempted to use an experimental weapon designed to prevent the tanar'ri from summoning aid. The tanar'ri ultimately won thanks to their Cambion spies.
Hellbound: The Blood War