I'd argue the Circle Tower is just as long, if not possibly longer, than the Deep Roads and contains two Big Bosses, numerous mini bosses and a high volume of combat. Yet it is the most often suggested area to visit first.
The Brecilian Forest contains over six maps, the same amount as The Deep Roads. And has numerous encounters, including a fight with a dragon (the only non-High dragon fight you can encounter, when the game only lets you fight a High Dragon in Haven or confronting Flemeth, both opportunities that are follow ups visitng one of the Primary Quest hubs, all but guaranteeing you are at least at mid-game) as well as a stiff end-game fight with either The Lady of the Forest or Zathrian.
And Redcliffe is often criticized because of its high skill checks and difficult combat as being a terrible place for a character like Alistair to suggest the player visit first, since it is often a challenge they are not able to address with a high rate of success.
And that's one thing that I like about DA:O - sufficient challenge at every step of the way. I'd like it even more if the entire game was given the Redcliffe treatment, when the player encountered many things in every instance they might not be strong or skilled enough to overcome. Which it seems DA:I is doing, so that's very cool.
Hmm. I can kind of see your point with the Circle Tower, if only, ONLY because of the Fade sequence. Not that I particularly mind it, but in my mind it's the difference between a relatively short quest to an uncomfortably long one.
And maybe I've just gotten too good, but I've never really had much trouble in the Circle (outside of the Sloth demon casting "blizzard" and my [Aggressive] characters not moving out of AOE). And in my opinion, the only difficult fight in the Brecillian Forest is/can be Zathrian. That dragon isn't very hard at all. Again, maybe I'm too good by now, though I don't power-build or anything.
I don't think Brecillian Forest qualifies as a dungeon because the areas are rather small, and at any one time you're rather close to the exit to the Dalish camp. Similar for Redcliffe--it's a two parter, you don't have to push through all at once.
I would say Redcliffe is indeed one of the more difficult--one reason being the obligatory Revenant fight, another being the fights where you have to protect all the NPCs like Murdock who're made of wet tissue paper (that fight, that night fight, is worth having two mages, both with heal, to keep those wimps around).
I dunno, The Deep Roads always just felt more like a slog to me. Buncha caves, going a linear direction the entire time, the boss fights...maybe I don't like Darkspawn. Maybe that's it. Probably the caves, actually.