I thought this was a really interesting episode, by Merlin standards. I appreciate its attempt to not conform to the standard narrative of most "filler" episodes, by introducing some one-dimensional baddie with unknown motivations who wanted to kill Arthur and whom Merlin would come to miraculously defeat within the forty-five minute time frame. Instead, I would argue that this was very much a character-driven piece, deriving its drama from the relationships between people, and that it did a fairly effective job of setting up the conflict that would unfold in the next two episodes. I think more "filler" episodes like this one--episodes that demonstrated some sense that the writers were indeed planning ahead--would have benefited the show tremendously.
I adored Mithian's character. Before she uttered a single word, I had an immediate sense that she was a no-nonsense princess, regardless of her beauty. As the viewer, wordless first impressions such as these are wonderful. Janet Montgomery did a fantastic job.
As far as Mithian and Arthur's engagement, yet again, Merlin knew better--apparently. To speak of this, and the ease with which Arthur set aside his promise to Mithian in favour of the absentee Guinevere, I would only be reiterating a point that has already been written of on countless other forums. Arthur's heart triumphed over his duty to Camelot. Romantic, but ridiculous. Not only was this aspect of the episode incredibly unrealistic, but it did Mithian's character a great disservice, morphing her into a tool, a victim of both Merlin's relentless intervention and Arthur's undying love for Guinevere.
One of the most illuminating lines of the episode, in my opinion, was Gaius questioning Merlin's arrogance. If only this had made some sort of impact on Merlin. He was basically having a temper tantrum the entire episode. It was very unappealing and very un-heroic.
I had high hopes for Helios here. During his conversation with Guinevere in the cave, there was something in his attitude, his manner of speaking to her, that made me think, for a split second, that there was much more to him than meets the eye. Yes, he was technically holding her prisoner, and we already knew him to be capable of slaughtering innocent people... but there was a kindness about him, almost. He fed her, he gave her a place to sleep, he clearly didn't order her to be restrained, she was allowed to wander freely. Perhaps I'm being generous in my interpretation of him. I was hoping he would be fleshed out further, that maybe we'd learn his motivation behind helping Morgana, or that maybe he'd turn out to be one of those baddies who'd redeem himself in the final moment... but alas, no such luck. By The Sword in the Stone, he was the one-dimensional villain.