The Dungeons beneath Chateau Haine serve as a convenient place for the Duke to deposit any criminals, malcontents or odd statues as he deems fit.
Background
The fortress was modified during the Fourth Blight and the dungeon passages were built by the Grey Wardens to help the citizens of Orlais escape the encroaching darkspawn. A crack in the wall within The Dungeons leads through a ghast infested cave-like passage out to the Retreat.
Involvement
Map of the area
Quests
The Du Lac Sign, if Aveline is in the group
Literary Theft, if Varric is in the group
The Odd Ensign, if Carver is in the group
Pirates and their Curses, if Isabela is in the group
A Secret Formula, if Sebastian is in the group
A Symbolon, if Bethany is in the group
A Warrior's Legacy, if Fenris is in the group