Stuart Wilson, an archaeology graduate who was working in a toll bridge booth, took a gamble and bought the field for £32,000 when he could have been investing in his first house.

Over the past 15 years he and a hardy band of volunteers have painstakingly unearthed what they believe are the remains of a sprawling medieval city.

Wilson, 37, is preparing for an exciting 2017. When the weather improves he and his colleagues will continue their work on the site, focusing particularly on what he says is the skeleton of a moated manor house.

He is applying for planning permission for an interpretation centre to tell visitors about the lost city of Trellech, and for a campsite for tourists and helpers.

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Wilson gave up his job so he could focus on the dig. Over the years he estimates that about 1,000 people have joined him, from archaeology students to intrigued people from the area.

He said he had so far pinpointed eight buildings, adding: “We’re finding building after building after building.”

A manor house, thought to have included two halls and a courtyard, may be the most spectacular but Wilson is fond of a well that was unearthed containing pieces of wood, bone and leather. Other features discovered include parts of jugs, cooking vessels, fireplaces and drains.

One of the best finds last year was more ancient: a flint knapping kit thought to date back to neolithic times. “I’ve asked the person who found that to come again,” he said. “They are clearly lucky.”

Wilson said the city was once home to about 10,000 people, perhaps a quarter of the size of London’s population at the time. He said the settlement was founded by a family called De Clare in the 13th century to manufacture weapons, armour and other military equipment.

Agricultural workers were tempted into the city by the prospect of more lucrative employment. “If you’re working in the fields you are living hand to mouth every single day – it’s a really hard existence,” said Wilson. “Suddenly, a big industrial town comes here, this is a great opportunity for you. You up sticks and move to the industrial town where the opportunity is.”

Most of the buildings, according to Wilson, seem to date from when the town was reorganised and built in stone after attacks by both English and Welsh forces. Evidence of the earlier town has been found below some of the buildings and occupation on the site may have started 100 years before that again, he believes. 

The city did not last long as a major centre. It was attacked by enemies of the De Clares and ravaged by disease. In later centuries fighters led by the Welsh leader Owain Glyndŵr targeted Trellech and it fell into ruin.

Wilson’s project has not been cheap. He estimates that it may have cost as much as £200,000 over 15 years. It has been funded by individual donations, commercial activities such as “experience days”, and through an online shop selling historical documents.

Wilson, who lives in a converted toilet block in Chepstow, believes that unearthing the city of Trellech will be his life’s work. “I think we’ve only found 0.1% of it,” he said.