Caska was a city in the Deep Nothing in D'Hara. It was the home of both Jillian's people and a central site. Before the Great War Caska was a thriving and beautiful city, however, it was reduced to rubble during the war and remained that way for the next 3000 years.
It was here that Richard came across the book Chainfire, proving the existence of Kahlan to his allies.
Caska was later captured by the Imperial Order. Emperor Jagang fooled the free Sisters of the Dark, into venturing there, thinking they were to meet up with Sister Tovi. Instead they were recaptured by the Emperor, whose ability as a dream walker had never failed to hold sway over their minds as they had believed.