The dungeon is a part of the castle that is used to house prisoners.
After being captured from her ice palace, Elsa was brought to the dungeon. Unconscious at the time, she was given a blanket and pillow while she recuperated on a bench inside her cell. As a precautionary measure against her magic, Elsa was chained to the center of the cell with her hands immobilized by a set of metal gauntlets. After seeing what she had brought upon Arendelle from the dungeon window and hearing that Anna was lost outside, Elsa grew fearful, resulting in her powers manifesting themselves uncontrollably. Initially, only the gauntlets were affected by Elsa's magic, but eventually, the walls of the dungeon took on a coating of ice.
When Hans arrived with a group of Arendelle castle guards to apprehend the queen, he found that the cell door had been frozen shut by Elsa's magic. Elsa managed to pull herself free of her restraints, breaking the gauntlets apart in the process; the structure of the dungeon was soon compromised, for the weight of the accumulating ice proved too much for the walls to bear, enabling Elsa to make her escape through the resulting hole.
The dungeon is a poorly lit room, with light passing through just a window at the end of the cell and the bars of the wooden cell door. The walls are comprised of stone, with archways built into them; each archway has a metal loop fixed at its center. A fixture in the center of the room can be used to chain prisoners in place while still allowing for limited movement around the cell. Two stone beds that resemble benches are placed diagonally from each other, with one near the door and the other near the window. The dungeon has a high ceiling with a series of wooden beams running along its width.