There are 2 locked doors on the western side of the dungeon, which is not related to any quests shown above.
There seems to be the "Gastyr Key" in the game file, it's just a guess, but didn't appear in the actual game, making those two doors totally inaccessible.[needs in-game verification]
The "Gastyr Key" when added to the game via cheat, does not open the door.
There is a chest behind one of those doors...
When the doors are bypassed via the developer's console, the chest contains only random loot.
There are SIX pressure plates in the prison where TWO Prismere troll is caged (Hard Lock). 4 round the centre of the room and 1 in each alcove between the cages. The cell doors open and close as you run over them. This will release the 2 Prismere trolls.