I have watched Merlin up through Season 5 - Episode 6.
Throughout this series I have seen two references to one of the characters (Morgana) being chained to a wall at the bottom of a well, with her dragon.
The first time it showed it in a nightmare that she was having.
The second time was in this episode, where she told Guinevere that she had been chained to a wall at the bottom of a well for two years.
Throughout the series these are the only two references to this event. I never saw it happen, or heard about it besides these two times. However it must have happened.
My question is, at what time period in the series did this happen, and the second question is who did it to her and how did she escape?