The first Game of Thrones stage spin-off has been launched by the Royal Shakespeare Company, and the verdict from most theatre critics is that it’s a worthy addition to the epic fantasy franchise.

Game of Thrones: The Mad King, a prequel set shortly before the events of George RR Martin’s first book and the all-conquering TV series, officially opened at the RSC in Stratford-upon-Avon this weekend.

The Telegraph awarded four stars to the “spellbinding spectacle”, while the Guardian’s four-star review said it features “spectacular stagecraft and plenty of modern resonance”.

The Times was more lukewarm, giving three stars and saying the play “lacks pace, sex and gore”.

The Game of Thrones origin story is epic in both ambition and execution – it has taken seven years to reach the stage, clocks in at three hours 45 minutes, and features a 36-strong cast.

The characters include younger versions of familiar figures like Ned Stark, Jaime Lannister and Robert Baratheon; and others whose fates were key to the events in the main saga, such as Lyanna Stark, Prince Rhaegar Targaryen and the “mad king” himself, Aerys II Targaryen.

After opening in Shakespeare’s home town, the RSC will hope it can go on to replicate the success of other spin-off plays like Harry Potter and the Cursed Child and Stranger Things: The First Shadow, which have both had lengthy runs in the West End and on Broadway.

The Shakespeare connection stems from the fact Martin’s novels were heavily inspired by Shakespeare’s history plays, especially those about the battles between the houses of Lancaster and York during the Wars of the Roses in the 15th Century.

Game of Thrones’ US creator was heavily involved in the play, stationing a representative in rehearsals at all times, and he attended the opening night in Stratford on Saturday.

In his review, the Guardian’s Mark Lawson wrote: “Purists will object that, rather than staging Shakespeare-adjacent populist spin-offs, the RSC should be offering its house dramatist’s royal dramas plain.

“The counter argument is that hitching on to a cultural juggernaut may bring in new sources of income and audiences.”

Audiences will respond differently to the play depending on whether they are already loyal Game of Thrones fans, which Lawson’s four-star rating reflected.

“This feels like a five-star production for most devoted throners but a three-star for those who haven’t seen the TV series,” he said.

Metro’s critic Dave Freak agreed, also bestowing four stars.

“As a stand-alone piece of theatre, The Mad King scrapes through,” he wrote. “But as an extension of the Game of Thrones world, it scores well, shifting the narrative to the stage with suitably theatrical thrust.”

The Telegraph’s Dominic Cavendish wrote: “At almost four hours, it’s a long evening, sometimes too dense with detail, and it lacks the grit and gore (not to mention the nudity) of the TV series.

“Still, it capably serves newbies and devotees, and speaks in a timely fashion to our age of unruly leaders.”

He added: “It’s not up there, yet, with the Harry Potter or Stranger Things prequels but, on balance, the Game of Thrones franchise has yet another worthwhile and lucrative spin-off on its hands.”

The Times’ critic Clive Davis had greater reservations, writing that while the play “gives hardcore GoT fans a chance to go hunting for ‘Easter eggs’, it seldom comes close to matching the grim aura of the TV series”.

“Duncan Macmillan’s script is sorely lacking in poetry. Events plod forward, one after the other, and although Michael Shaeffer is a thoroughly compelling presence as the cruel and chronically unstable monarch Aerys, other performances seem nuance-free. Too many lines are simply shouted. You expect better of the RSC.”

Davis concluded: “It goes without saying that there are more than enough GoT devotees to keep the box office ticking over for the rest of eternity, but only the most dedicated of them will have the energy for this marathon.”

Others were more enthusiastic, and Whatsonstage’s Sarah Crompton’s five-star review said the play is “a triumph; a soaring, surging spectacle and a latter-day history play, both serious and popular”.

She added: “It is a remarkable achievement, wonderfully directed by [Dominic] Cooke and superbly acted by every member of the 35-strong ensemble. The result is a brilliant exhibition of how theatre can create magic.”

The Independent’s Alice Saville, awarding four stars, said the “stellar stage show has all the thrillingly staged battles, gory deaths and, yes, dragons that the most bloodthirsty fan could hope for”.

“But there’s also a surprising amount of heart – of the warm, still-beating kind – as it lingers on the faces of its fallen,” she said.

The play begins more than a decade before the events in the first book and TV series, opening with the tourney at Harrenhal, a great jousting tournament where the houses of the Seven Kingdoms gathered and where key events in the Game of Thrones legend took place.

Macmillan said he wanted to create something “on the scale of a Shakespearean tragedy and a Shakespearean history”.

The playwright told BBC News: “The challenge was writing something for a fan base who take this world and their characters very, very seriously and know so much about it, and also the complete newbies, and writing something that is going to work for both of those people.

“Making something that is going to knock the socks off of everybody has been a real challenge. It’s taken a long time, but I think we’ve got there.”

Cooke said there were “lots of similarities” with Shakespeare, especially themes of “dynastic struggle, what it means to have power, intergenerational conflict and powerful families”.

Martin has been “very supportive” in the process of creating the play, the director added.

“He’s been collaborative in terms of the development of the script. We’ve worked very closely with him. And George, I think, picked up quite quickly that we were very serious about honouring what he’d done.”

Tamara Harvey, the RSC’s co-artistic director, said: “George himself talks about how Shakespeare and the Wars of the Roses inspired him to write these stories.

“So the connections feel very deep, and of course themes are shared: power, love, succession, battles for who gets to rule who – it’s all there in the Shakespeare history, just as it is in Game of Thrones.”

Asked about future plans for the play, Harvey replied: “We’re hoping as many people as possible get to see it, and that starts here in Stratford. And from then, who knows?”