The first surprise came in February 2015 when the people of Cornwall woke up to find someone had carved a face into the rock below Tintagel Castle. Was it the act of vandals, determined to desecrate the most ancient and spiritual of all Cornish places, the very home of King Arthur? In fact, it turned out to be the work of local sculptor, Peter Graham, commissioned by the castle’s custodians, English Heritage.

The sculpture of Merlin’s face at Tintagel in Cornwall. Photograph: English Heritage

Now a second development – or outrage, according to some – is afoot. English Heritage is planning to dig under York’s most iconic of locations, the 12th-century Clifford’s Tower, and build a visitor centre. The £2.5m proposal went through the council’s consultation period in January 2016 when the city was engaged in cleaning up after a devastating set of floods. (This inundation, ironically, had been so bad that it had actually engulfed the Tower.) Now, after a crowdfunded challenge by independent city councillor, Johnny Hayes, that planning process is subject to a judicial review at the high court.

“In every way this proposal is wrong,” said Hayes. “When I heard it, I thought it was a bad joke.”

Clifford’s Tower is all that remains of York’s royal castle, the seat of power in the north from the time of William the Conqueror. He had constructed the mound on which Henry III later put the French-style four-leaf-clover tower. Subsequent kings held parliaments here and, although the interior was burned out in the 17th century, the exterior walls, with their improbable cracks and leans, have become a much-loved emblem of the city, attracting around 150,000 visitors annually. Together with the medieval walls and the Minster, the tower is seen as a vital part of historic York.

The planned changes, drawn up by London-based architects Hugh Broughton, are to revamp the interior of the tower, an area that most people agree has been a disappointment to visitors, and create a covered spiral walkway that will reveal parts unseen. It is the plan for the gift shop and interpretation centre, embedded under the mound, which has aroused local opposition, including a 3,780-signature petition.

Artist’s impression of the visitor centre at Clifford’s Tower. Photograph: English Heritage

“It would be like a boil growing on the side of what is a unique urban setting,” said Russell Wright, local resident and former research fellow at the Institute for Advanced Architectural Studies. “Would they dare to do that to the Minster?”

Jeremey Ashbee, head curator at English Heritage, disagrees. “The section of the mound we will use was put there in the 19th century – it’s not original. And with this project, we have a chance to tell the Clifford’s Tower story properly.”

These are interesting times at English Heritage, an organisation that has never previously been known for courting controversy. Started back in 1949 as the Ministry of Works, it soon amassed a portfolio of ruins and other buildings that attracted increasing numbers of visitors – 5.5m a year by 1970, making it the largest manager of visitor attractions in the UK.

Then in 1983, the Conservative government decided to transfer the entire collection to a new body called, after some tinkering, English Heritage. In April 2015 that organisation became a charitable trust and was handed an £80m lump sum, around a third of which is for capital investment. “It’s an opportunity to be more imaginative and a bit bolder,” said Ashbee.

Clifford’s Tower is the first site to benefit, or suffer, from that new investment, but others are soon to follow. Framlingham Castle in Suffolk is to get a £1.2m makeover while at Tintagel a £4m bridge is planned. Judging from the Merlin’s face furore, and now the judicial review on Clifford’s Tower, the change to a bolder style of heritage may yet end in ruins.

• This article was amended on 2 February 2017. An earlier version said the Clifford’s Tower proposal was passed by the council in January 2016. A consultation period was held then; the proposal was approved by the council in October.