Inside one of Britain's most magnificent medieval relics

It rises from the flat fields on the south bank of the Humber Estuary – a magnificent, towering building that, at first glance, resembles a castle.

"We are looking at, in a nutshell, the largest and most magnificent abbey gatehouse in the British Isles," says Dr Alastair Oswald, a historian with English Heritage.

He is showing us around Thornton Abbey in North Lincolnshire, which was once one of the country's richest Augustinian monasteries.

Only ruins of the abbey remain today, but the gatehouse survives with much of its splendour intact, as the Secret Lincolnshire podcast has been hearing.

Sadly, it has been closed to visitors for the past five years due to the dangers of falling stonework, but this summer guided tours have resumed on a monthly basis, with more dates due to be announced for next spring.

Thornton Abbey was founded by William le Gros, Count of Albemarle, in 1139. He had defeated the Scots at the Battle of the Standard, in North Yorkshire, and was "in effect the king in the north", according to Oswald.

"This was the first of a string of religious houses he founded as a kind of celebration of his own power.

"Today, it feels like you're a long way from anywhere, but actually there's a little beck about half a mile from where we're standing which flows into the Humber Estuary. And we know they were using that beck to transport food and drink and stone."

Fragments of the abbey remain, including part of the chapter house that features intricate carved stonework.

But it is the gatehouse that dominates the site today. Work on the building started in 1377 and it took 12 years to build.

Inside is a spiral staircase, narrow passageways and nooks and crannies, including a medieval toilet. The most stunning space is a large room with a grand fireplace and a window.

According to Oswald, few other places offer such an experience of maze-like, little passages and dead ends, because so many medieval abbeys have been reduced to rubble.

"Whereas here you've got the potential to really explore these nooks and crannies which you don't normally find," he adds.

The gatehouse offers just a flavour of life in medieval times.

"What we lack is the sort of paintings and colour and the carpets and so on that would have been here – the hangings on the wall, the smells of roasting meat and the burning wood in the fireplaces, and so on.

"But still you do get that sense of what it must have been like in parts of this building that you don't get anywhere else in the country."

That extends to the graffiti that covers the back wall of the gatehouse. While some is from the 19th Century, depictions of two ships are "classic medieval types".

"They are medieval religious graffiti, in fact, because the ship is an emblem of religious life," says Oswald.

"They do illustrate that important link with the sea that Thornton Abbey had through its trading links."

Unsurprisingly, some lurid legends have become attached to the site, such as the story of an abbot who is said to have been found walled up there.

"In some versions he's sitting at a desk with a sort of quill grasped between his bony fingers and a little candle that's burnt right down to the bottom on the desk in front of him," Oswald says.

"I think the whole thing's a myth. But there are, as we've seen, these little tiny spaces where you could imagine that myth arising from."

For Kimberley Royce, the site's keyholder and former manager, the abbey has been her life for the past 20 years.

"It's a magical place. I can't think of working anywhere else," she says.

"When I first came here, I didn't actually really know I wanted to be here because I came from a city, and it took me three months to realise that I hadn't actually made a mistake, and I've enjoyed every single minute of it."

She adds: "The gatehouse is just magical. You don't expect to see what you find in there."

It also appeals to children, she says, because it resembles a castle.

"We used to have families coming and having picnics in the grounds," Royce says.

"It was buzzing, and people miss it. People turn up and they want to know why it's shut. So I wish I had a pound for everybody who asked me that question.

"But we will get there in the end. We haven't got anything like this in the country."

Listen to more episodes of Secret Lincolnshire on BBC Sounds.

Tours of Thornton Abbey resume in the spring, but other medieval sites in the Humber region can be visited during Heritage Open Days, which start this weekend.

Here are three to visit.

Beverley Friary: This former Dominican friary in Beverley, East Yorkshire, was founded in 1233, and while the church and cloisters now lie beneath a railway line and road, surviving buildings include what are thought to be the friars' library and dormitory, with features dating from the 15th, 16th and 17th Centuries. Open Saturday and Sunday, with no booking required.

The Bayle, Bridlington: A Grade I listed building dating back to the 12th Century, when it is thought to have been a gatehouse to a castle built by William le Gros, who also founded Thornton Abbey. It was later used as the gatehouse to a priory, as well as a prison, a court and a schoolroom. Open 19 September, no booking required.

The Charterhouse, Hull: Founded by Michael de la Pole, Earl of Suffolk, in 1384 as a monastery and almshouse for the elderly, the Charterhouse continues to provide social housing today. It was flattened during the English Civil War and rebuilt in the 17th and 18th Centuries. The poet Andrew Marvell played in the garden as a boy. Open 19 and 20 September, no booking required.

Listen to highlights fromLincolnshire on BBC Sounds, and watch thelatest episode of Look North.

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