It’s bold, it’s controversial, it’s a £3m footbridge reuniting two ends of a medieval castle that gave birth to the myths of Merlin and King Arthur. Now, after a delay caused by bad weather last week, Tintagel Castle will finally reopen to the public. Visitors will be able to walk between the two ruins, separated when the original land bridge collapsed several centuries ago.

The project has seen English Heritage accused of “Disneyfying” one of the most beautiful sites in Britain, with the Cornwall Association of Local Historians expressing outrage that the head of the wizard Merlin has been carved into a rock face – proof, for them, that the charity has attempted to turn the site into a “fairytale theme park” devoted to the legend of King Arthur. (And don’t even get them started on the Arthurian bronze that towers over the headland.)

Visitors at Tintagel, ahead of the bridge’s opening this weekend. Photograph: Jim Holden/English Heritage

But chief executive Kate Mavor has hit back, denying the project is dumbing down history. “It’s not Mount Rushmore. [The Merlin carving] is really small and actually really hard to find,” she told the Observer. “The generation coming through now are constantly stimulated by imagery, and if you just write something down and say ‘read this and then imagine’, it’s very hard. If you just put a few interventions very carefully and sensitively … you can get people excited by history.”

Local enthusiasm for the bridge appears to have slightly dampened. “It looks like the golden gate in the middle of a medieval ruin,” said one resident. “It doesn’t exactly fit, does it?” But the structure, which uses engineering honed in the Alps and was designed in a competition won by engineering firm Ney & Partners and architects William Matthews, has drummed up huge interest. The response from tourists has been overwhelmingly keen; fed up with constant questions about when the bridge would open, one hotel reception posted a notice directing guests to the beach.

You won’t please everyone. We try to stick to what we are for, and that is bringing history to life Kate Mavor, English Heritage

The 68-metre bridge will mean visitors are no longer faced with climbing 148 steps to get round the popular tourist site. It is one of the most ambitious, complicated and at times controversial heritage projects seen in Britain in recent years. But, as it opens, Mavor is battling to maintain the organisation that made it happen.

How will English Heritage credibly survive? It’s an existential question as much as a financial one – the charity has to become entirely self-funding by 2023, when an £80m government grant runs out. Balancing the need to draw in large numbers of people and stay true to the organisation’s core purpose is the problem Mavor faces every day.

“You won’t please everyone. We try to stick to what we are for, and that is bringing history to life and basing everything on thorough research and evidence,” she says.

Mavor makes a strong case for audiences understanding the past to navigate the future. “Being connected with the past is really important. Research shows that wellbeing improves when you can see yourself as part of a whole. Belonging and purpose are the two biggest things to feeling a sense of place,” she said.

English Heritage chief executive Kate Mavor on the new bridge. Photograph: Emily Whitfield-Wicks/English Heritage

Yet English Heritage, much like the National Trust, has been accused of enforcing a narrow and prescriptive take on English history that has often erased the contribution of women and of black and Asian Britons. Activist and journalist George Monbiot railed against the heritage industry on Thursday, tweeting: “Curators of stately homes leave us, in most cases, entirely ignorant of the enclosure, expulsion, murder and slavery on which they were built.” How important is broadening English Heritage’s remit?

“To be under-represented is something we really understand,” said Mavor. “We have started to address that and are specifically looking for examples of history that haven’t been told.” She points to Portchester Castle in Hampshire, which in the late 18th century held over 2,500 African Caribbean prisoners of war, as well as the appointment of Professor David Olusoga to English Heritage’s board.

She is excited by a forthcoming podcast in partnership with comedian Josie Long on women’s history as another way to help achieve this. “That’s the joy of online, because there is so much you can do without having to invest in the bricks and mortar of buying a site. Already if you look online you will find we have information about people in history who have been under-represented.” Except it’s not as interactive or physical as actual sites under English Heritage’s care.

“No it isn’t, I completely agree … We’d love to be the sort of charity people support because they see their stories represented and in places.”

Last year, there were 6.3 million visitors to English Heritage sites, which include 66 castles, 53 Roman sites, seven palaces, three deserted medieval villages and one cold war bunker. Stonehenge is the biggest money-maker, providing a quarter of admissions income, and Hadrian’s Wall is one of the most visited. The organisation also looks after London’s blue plaque scheme, which honours notable figures in English history.

“Back in the day we were accused of being too worthy and dull and focused on the academic, so there will always be people who see things in a different way when you’re trying to appeal to the whole nation and looking after the nation’s assets,” said Mavor.

The Rausing family, heirs to the Tetra Pak fortune, donated £2.5m to make the Tintagel project happen after it was mentioned to them by English Heritage chairman Tim Laurence. “They were very excited by it and they will give us further money later in the year because they’re very enthused by what we do,” said Mavor.

New walkways have been introduced along the cliffs and the excavation has uncovered more pottery from the Mediterranean than any other site in Britain. Its curators say it was “a party palace” for Richard, the first Earl of Cornwall, who owned 40 castles across England in the 13th century. “We’ve been hurt by accusations that we’re selling out, there is no justice in them,” said Jeremy Ashby, head curator.

Tintagel remained a triumph, said Mavor. “Our millionth member signed up to English Heritage here, at Tintagel, last year. We’re proving we can excite and inspire people around history.”

• This article was amended on 11 August 2019 to correct the spelling of Tim Laurence’s name.