Warning: Full reviews contain spoilers for those who have not yet seen the episode. Updates will be made as more reviews come in.
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Den Of Geek "Colin Morgan manages to continue his strong performance as Merlin, managing to ride the gamut of emotions through the episode, while Bradley James retains his dignified standing despite some rather silly moments. John Hurt, as the voice of the dragon, gets a few moments to shine. This must be the easiest job of all time for a renowned actor."
Radio Times "It’s a hearty broth of a story, with a stirring, widescreen quest, gigglesome asides, a lovely, unspoken ice-breaker between Merlin and his mentor, and a story arc that hits the spot - younger viewers will adore the ending."
Daemon's TV "Last week Arthur was crowned king of Camelot after Uther’s death, yet if you expected to see much of Arthur settling into his golden hat, you were mistaken as this episode mostly leaves Camelot by the wayside and Merlin embraces his oft mentioned, underused title Dragon Lord."
Dan's Media Digest "The big problem I had with this episode was very simple: it just wasn't anything to get excited about. The show has a few templates it likes to use, and this one contentedly went about its business and delivered no major surprises. It's quite dispiriting when a show squanders its potential like this, as there writers out there who'd love the opportunity to see their imaginations come to life on-screen, so why isn't every episode trying to do something fresh and interesting?"
Clique Clack "This is the first episode of season four that has been filler, and I have to say, if this is going to be the state of filler for the rest of the season, we could very easily reach season two levels of campy mindless joy, if not season one levels."
The Digital Fix "After a few weeks of doom and gloom, this is more like the Merlin we all know and love. It goes right back to the early days when Gaius would warn Merlin not to do something but the Dragon would tell Merlin he had to do it, so he would do it and things would go tits up and he’d have to sort everything out."
Critictoo "If Arthur was crowned king at the end of the previous episode, the withdrawal for over a year of Uther has resulted in little change to occur. Still, Merlin is determined to announce happier times for practitioners of magic. For now, it's all the right things to earn more than anything."
Dork Adore "After three bleak episodes, I was wondering whether Merlin was hellbent on being miserable this year. Don’t get me wrong, none of those were bad, but nor were they happy, and the zippy bantering unity is part of this show’s appeal - that and Bradley James topless, obviously. So, this week, something more traditional, perhaps? But will it seem hopelessly outdated?"
Digital Spy "'Aithusa' seems like an important instalment in terms of Merlin's overall plot arc, but the 'quest' element of the episode does drag a bit - Arthur, Merlin and the knights seem to walk, stop, make camp and then carry on, ad infinitum."
Geek Syndicate "So the time has come for the first filler episode of the series! It couldn’t all be dramalama in Camelot now could it? A mysterious stranger named Julius Borden (Battlestar Galactica‘s James Callis) arrives at Gaius’ door searching for help to gain access to the last part of the key to the tomb of Ashkanar. Why is it so important?"
SFX "This is a fun episode to balance out the darkness of the past three weeks, but it’s a shame the enormity of recent events is completely swept aside in favour of a simple A to B quest. It’s weird that people aren’t more glum about the absence of Lancelot, and why does nobody mention Uther this episode? Surely the coronation of a new king is the most important thing to happen in decades?"
Cultbox "The first episode of this brand new era of Merlin could have explored any of the aforementioned quandaries and been a hugely engaging piece of drama, but instead Aithusa focuses on a wholly uninvolving plot regarding a dragon egg and a rather bland character we’ve never met before called Julius Borden. Apart from a few passing references to Arthur’s new monarchic status, this episode could realistically have been placed anywhere within the last two years of the Merlin canon without any major continuity problems."