It's one of the most famous, most studied, most archetypal passages in epic poetry: in Book I of Virgil's Aeneid, the Trojans, exhausted from their voyage and desolate at the loss of their city, gaze down on the city of Carthage as it rises from the African soil, its people scurrying like worker bees in their manifold tasks. It provided the high point in the Royal Opera's new staging of Berlioz's magnum opus Les Troyens last night: a brightly costumed chorus singing down from a terraced city carved into a red sandstone cliff, inspired by views of Morocco.

You get a lot of opera for your money in Les Troyens - a shade over four and a half hours of music plus an hour of intervals. If we're all being honest - and I feel the wrath of Berlioz's ghost breathing down my neck as I write this - you get two operas: a taut, dramatic two act piece about the fall of Troy itself, followed by a more elegiac three act piece telling the story of Dido and Aeneas in Carthage. Only the latter was performed in Berlioz's lifetime, the total work having been thought to be too unwieldy even by Berlioz's friends and admirers.

The initial two-act piece is told principally from the viewpoint of Cassandra, the prophetess doomed to be disbelieved. It makes for an interesting angle on a story where we all know what's going to happen, especially so because Anna Caterina Antonacci gave a blistering performance. Antonacci is a true dramatic soprano: she has both the stage presence and the vocal power needed to convey the sense of urgency and despair as Cassandra fails to convince anyone to do anything other than, ultimately, join her in suicide. Fabio Capitanucci gave us warmth and similar power as her betrothed Coroebus, and it was a shame that both characters had been killed off by the end of Act II.

The main role which spans both sections is that of Aeneas, sung by Bryan Hymel, called in to replace Jonas Kaufmann. Hymel is a fine lyric tenor, who sang beautifully in Inutiles regrets, the Act V aria in which Aeneas is being torn apart by the conflict between his love for Dido and his duty to go off to Italy and found Rome. But for the first four hours, I felt he was miscast. Aeneas is called upon to make huge, heroic entrances, dominating the stage: the role requires a the firepower of a Wagnerian heldentenor or something close to it, and Hymel simply didn't make an impact on that level.

The chosen period setting was in Berlioz's era (the programme notes show photos of the siege of Sebastopol in the Crimean War). Es Devlin's sets for Troy were grey and metallic; lighting was muted. It was all rather dingy: if you think that it's effective to mirror sad events by drabness of staging and lighting, you'll approve - it didn't do much for me. The Trojan Horse was suitably impressive, a massive fire-breathing scrap metal sculpture, although when it rocked to and fro as it was moved onto the stage, I was unable to suppress the uncharitable thought that it looked like a giant nodding dog.

Berlioz's music is full of interesting lines and bursts of orchestral colour. But for such a long evening, it felt rather disjointed: there seemed to be little sense of forward progression in the music or direction in the way it followed the drama - although, admittedly, it's the first time I've heard Les Troyens, so I may perhaps get a more integrated feel of the work on future listenings. The orchestra's performance, under the baton of Pappano, struck me as no more than workmanlike: there were plenty of good moments, but only a handful of occasions when I was really lifted out of my seat. It's a work that will be unfamiliar to most of the players, so I expect it will improve as the run proceeds. It improved through the evening, and the music to the great Act IV love duet was stirring.

And so on to North Africa, with the spotlight turning to Eva-Maria Westbroek as Dido. Westbroek sounded lovely, with rich generous timbre and effortless stretch to both low and high notes, somewhat at the expense of diction. However, I thought she was oddly directed by David McVicar. In Act III, Dido is supposed to be a proud queen bravely recovering from her husband's murder and establishing herself in exile. Rather, her body language was that of a carefree, spirited maiden who is charming and delightful to all around her - more Queen of the May than Queen of Carthage. Westbroek was altogether more credible in her bursts of impassioned rage and their aftermath: the scene as she agonises over his departure with her confidants was impressive, played on a completely blank stage.

I'm not personally a fan of the French Grand Opera tradition of including extended ballet sequences in operas, and Act IV of Les Troyens has several of these. The best music came in the most famous passage (the Royal Hunt and Storm), but I wasn't taken with Andrew George's choreography - ballet fans may differ. But I enjoyed an excellent comic relief scene between two Trojan guards early in Act V, and I was very taken with three fine lyrical singing performances from smaller characters. As the priest Narbal, Brindley Sherratt produced a gloriously rich bass legato; as Dido's court poet Iopas, Ji-Min Park produced a lovely lyrical pastoral hymn O blonde Cérès, and Ed Lyon sang a wonderful exile's lament.

Staging any opera on this scale is a magnificent achievement, and this production will probably keep getting better as it moves on to Vienna, Milan and San Francisco. I found this evening rather mixed - the Troy section genuinely dramatic, the Carthage section with some wonderful singing. And whisper it not: the two sections are so different that I suspect Les Troyens might work better as two operas on successive evenings (as was done prior to 1921) than as the single work as Berlioz conceived it. But this was a performance heavily sold out, mainly (I suspect) because it was the London opera season's only chance to see Jonas Kaufmann, and his appearance was sorely missed.