It's easy to take the mickey out of Merlin, the BBC fantasy series which returns to our screens on Saturday. Three series in and this revised take on the legend of King Arthur has slipped into the kind of comfortable groove that breeds cliche – I'd be very surprised if series four didn't feature Morgana flashing her dark eyes evilly, an important cast member being "hilariously" possessed by a magical creature and Richard Wilson's Gaius saying "Be careful, Merlin, it's dangerous!" on at least 3,000 separate occasions.

And yet I'm still horribly addicted to the show. Merlin may not be the kind of TV drama you would recommend to friends in the pub. It has little of Fringe's eccentric verve, or Boardwalk Empire's dark and stylish eloquence, but Merlin is outrageously watchable, occasionally ingeniously plotted and benefits from a remarkable – if sometimes overly matey – chemistry between key cast. Recent US rival Camelot might have been a lot racier and featured a genuinely classy villain in the form of the magnetic Eva Green, but nobody really cared a jot about any of its protagonists. By contrast, the cast of Merlin feel like old friends, albeit ones you occasionally wish might make a bit more of an effort.

The BBC series' simplicity lends it a fable-like quality, and its arrival each autumn is perfectly timed to coincide with thoughts of mulled wine and raging fires in the grate. Winter somehow feels like a far more British time of year than summer – and there's nothing more British than the tale of King Arthur, even if Bradley James seems more like a 1990s boy band member than the hero of Geoffrey of Monmouth's 12th-century tales.

Speaking of which, Merlin's been criticised for its "modern" appearance, by which I can only assume that people take exception to the number of black actors in the cast (unless it's the often cheap-looking CGI). This is a fantasy series based on a flowery medieval rendering of a Dark Ages hero who may or may not have existed. If he did, he probably lived in a hut, not a castle, and is unlikely to have ever taken part in jousts because they were not introduced in England until the Normans arrived here in 1066. Frankly, this is a story that took a sharp turn into the bounds of rampant impossibility a long, long time before it was decided to use a mixed-race cast. Besides, there may well have been black people in Britain in the post-Roman period.

I'm intrigued to see how the writing team take matters forward in series four. With Morgana apparently defeated, Uther incapacitated and the bare bones of the knights of the round table in place, it seems like the right time for Arthur to come into his kingdom, as the story shifts towards a more traditional rendering of the myth. Unfortunately, Merlin would be destroyed as a show were Colin Morgan ever to reveal his wizardly powers, since each episode riffs to greater or lesser extent on Arthur realising that the clumsy serf who's been washing his socks is in fact the mightiest sorcerer the world has ever known. Once that dynamic's gone, there's really nowhere for things to go.

It's also worth remembering that the story of Arthur is essentially a tragedy. It ends with the king betrayed by Guinevere and Lancelot and destroyed by his own blood relative (his son in many versions). If we want the show to continue as the perfect cosy companion piece to the changing of the seasons, as the nights draw in and the fire crackles, we'd be wise not to wish for the writing team to move the story on too far, too quickly. Merlin's optimistic tone could never survive the shift into a torrid tale of death and destruction.