At what point was Morgana chained to the bottom of a well?

In season 5 episode 8, Sarrum visits Camelot. Gaius and Arthur talk about a rumour that says Sarrum had Morgana under lock and key. It's easily missed, and maybe not that easy to put in connection to the capture Morgana speaks of. I think that's what happened though. I believe Sarrum is the one who had Morgan and Aithusa locked up.

When this happened and how she escaped is a mystery, however.

Well shite, apparently we get to know all of it in s05e08 XD

Apparently this got a bit confusing, sorry about that. I wrote my answer before fully watching the episode through. What I mean is; Arthur and Gaius talk about Sarrum having once had Morgana under "lock and key" in s05e08, but we don't get to know specifically if that is the same instance Morgana has a nightmare about and tells Gwen about. Later on in the episode Arthur and Sarrum speak about the fact that Sarrum once had Morgana under lock and key, but somehow she escaped. They don't say how and they don't say when this was. Since I've been binge watching Merlin these past weeks, Morgana's nightmare was fresh in my mind, and I figured that was the instance they (see: Sarrum, Arthur and Gaius) spoke of in the episode.

Alas, all I can give the OP is the answer of whom; Sarrum. That is, if they are speaking of the same instance. Hope this cleared things up a bit :) My mother language is not English, so apologies for any confusion ^^'

I am currently rewatching Merlin and have come across this same issue. Between season 4 and 5 there is 3 year gap, as stated by Arthur in season 5 episode 1 (at the very start of the episode Arthur says 'for three long years we have been blessed with peace and prosperity'), and since season 4 ends with Aithusa and Morgana together I would assume that they were both taken to the pit Morgana talks about shortly after. Unfortunately it is only mentioned briefly and they don't seem to give any concrete information on further details or who was responsible. So I'd say Aithusa and Morgana were in the pit for two years immediately after season 4 (which is when Aithusa would've been growing, thus becoming crippled), and then they escape at some point and somewhere in the next year or so make the allies we see Morgana have at the beginning of season 5. Hope this helps :))

Just saw this episode and the answer you're given doesn't seem to make much sense. Your question is on where on the shows timeline is morgana imprisoned to where she feels capable of guilting gwen with said information (one would assume this would have to be imprisonment in the past that has already been exposed to the viewer by S5E6.) Coming into gwen's meal with morgana cold they have clearly made a mis-step with their story arc, (haven't seen S5E8 yet so if they explain her imprisonment then its another example of bad writing that ultimately led to the show's cancellation, also morgana is projecting her own unrelated suffering on to gwen if it is completely unrelated to camelot, and even if it is related to camelot it isn't gwen's fault she is elevated above where a bastard like Morgana ever could have been )