As the Roman Empire fell and the legions withdrew to Rome, Britain was left to the mercy of native warlords and invading Germanic tribes. And owls.

At least that seems to be the case at one site excavated in Gillingham, where archaeologists believe an abandoned Roman mausoleum stood derelict but intact, looming over the surrounding land for more than 700 years – and was occupied by a population of tawny owls.

It's just one strange but fascinating insight to come from the archaeological dig at Grange Farm conducted by Pre Construct Archaeology, which is about to publish its findings in a 200-page study 'By the Medway Marsh'.

James Gerrard, senior lecturer in Roman Archaeology at Newcastle University, said the publication was the culmination of years of work following excavations off Lower Rainham Road in 2005 and 2006, prior to a house-building project.

While the mausoleum and its contents are intriguing alone, the everyday lives of the people who built it and lived nearby are even more enthralling.

Dr Gerrard explained investigators had found a huge amount of litharge – a lead oxide (PbO) and by-product of the 'cupellation' method of silver extraction – which means the inhabitants were melting down metal to get at its silver.

The 15kg of litharge found by the team is the largest amount ever uncovered at a Roman British site, which means the silver extraction was being done on what would have been an industrial scale for the period.

"Was that legal?" asks Dr Gerrard. "Was that supervised?"

As with many archaeological mysteries, those questions remain unanswered but it's known that the late Roman world of the 3rd and 4th Centuries AD operated on a gold and silver economy, and the control of those metals was closely tied to imperial taxation.

"Quite why people were refining silver from silver-rich base metal alloys is a mystery," said Dr Gerrard. "Quite what the objects being melted down were is a mystery too. They probably weren't coins, as the bronze coinage had too little silver in it. We might expect that the refining of silver here was either being done officially by the 'Roman state', or perhaps illicitly. It's an unusual aspect to the site.

"Maybe they were making silver objects like the ingots in the Canterbury Treasure."

The earliest evidence for occupation at Grange Farm occurs during the Late Iron Age, about 100BC, before the site grew into a small Roman rural settlement in the late first century AD, and the settlement evolved until the 5th Century AD when it was abandoned.

Metal extraction took place at one end of a building referred to as an "aisled structure" – a timber hall divided in three aisles by internal posts, which are commonly found in Roman Britain. These 'multi-function' buildings tend to show evidence of high status domestic use at one end, with fireplaces in the middle and evidence of craft work and metal work at the far end.

"There was probably quite a large group here," added Dr Gerrard. "These are people who were working the land, they're working on the marshes, they're doing some hunting, they're growing crops and raising animals, they're doing metal working."

But who were these mystery silversmiths?

While we can't put names and faces to those who lived at the site, the evidence from the site's mausoleum brought archaeologists another tantalising step closer to them – and one of them in particular, who was found in a lead-lined coffin.

Mausoleums from the period are more usually associated with Roman villas than 'aisled structures', and the existence of the monument – which would have stood at almost the height of a two-storey house – proves the occupant was a high-status woman.

"The mausoleum is a house for the dead," explained Dr Gerrard. "It's basically a funerary monument. It probably dates back to the late 3rd Century or early 4th Century AD, and it was a stone building structure, probably with a tile roof. It was probably quite tall – certainly visible from the Medway – perhaps about the height of a two-storey house or a little less.

"It's quite unusual in that it had a tessellated pavement of plain mosaic – a plain red colour – which is really unusual for Roman Britain.

"This middle aged to elderly lady was buried there in a lead-lined coffin. She was probably local from the isotope analysis we did on the teeth.

"The silver suggests wealth. The mausoleum is wealth. It takes resources to build the structure like the mausoleum and it takes resources to put someone in a lead coffin.

"She had quite a hard life though. She had osteoarthritis but she lived to a good age and was buried with reverence. I think she was quite high status. She was no peasant and she was someone with clout locally."

Further evidence of wealth comes from gold jewellery found in the rubble of the mausoleum – including a necklace or bracelet made of gold filigree double-loop links threaded with polyhedral faceted beads of variscite.

Evidence of wear and modification suggests it may have been a necklace turned into a bracelet for a child, and it's not known if it would have come from the mausoleum itself or sarcophagi possibly located next to it.

Unusually for such monuments, the mausoleum stayed intact until the 11th or 12th Century – and its afterlife provides more room for conjecture.

While we'll never know exactly what happened in those intervening years, it's fascinating to imagine the Roman funerary monument lying derelict for hundreds of years, visible for miles from the Medway and surrounding landscape.

"We think during the 5th Century the grave is disturbed," added Dr Gerrard. "We don't know why that was – and then the building stayed up until the Norman Conquest."

But evidence of pellets containing tiny rodent bones suggests the mausoleum wasn't left totally abandoned.

"We've got tawny owl pellets," said Dr Gerrard. "The building becomes ruinous and then you've got owls living here.

"It's the end of the Roman Empire, the mausoleum is abandoned and the owls take up residence – we can't be too precise about when that was but it would have been somewhere between the 5th and 10th Century."

Even more intriguingly, evidence suggests Anglo-Saxons who arrived in the centuries after the Romans continued to come to the site without taking up residence.

"This is where it gets interesting," added Dr Gerrard. "You're in Kent at the forefront of Anglo Saxon migration. We've got a similar number of Anglo Saxon objects – a 6th Century silver brooch, two Anglo-Saxon spearheads, a quantity of Anglo Saxon pottery.

"It looks like people are coming to this monument and interacting with it in some way but they're not living there and they're not burying their dead there."

So did the monument continue as a some kind of sacred – or cursed – site for the tribes who followed, or was there a more practical reason for its continued popularity?

Dr Gerrard has at least one theory. "If the building is visible from the Medway it might be a navigational structure for people coming down the river," he suggested. "It's the 5th Century and water was more important as a means of travel."

So while most Roman mausoleum's are abandoned and left to ruin, was this one maintained by Anglo-Saxons, as a landmark for sailors? With no real evidence that's a step of conjecture too far... but it's not beyond belief.

What's undeniable is the site is hugely significant in terms of archaeological value. The team uncovered 453 Roman coins, 20,000 fragments of pottery weighing a quarter of a ton, and 8,000 animal bones.

"This is why it takes 15 years to do the analysis and it's a 200-page report," explained Dr Gerrard. "It's definitely significant. It's really important for the late Roman period in Kent."

As for the mausoleum itself, its story probably ended sometime after Domesday in AD1086 when the land – recorded as having pasture, a probably tidal mill and six unfree peasants – was given to Bishop Odo of Bayeaux, half-brother of William the Conqueror.

"The site then becomes the medieval manor," added Dr Gerrard. "Probably what happened was they reused the stone from the building to build a chapel."

About AD1122 the manor was called Grenic, then Grenech in AD1198, Grenge in the 14th Century, more recently it became known simply as Grange Farm – referred to in the name of the nursing home on the site of the medieval manor house, which has become 'Grace Manor'.

Which brings the story of Grange Farm to an end.

For Dr Gerrard himself, and the rest of the team, publication of the findings also marks the end of an epic story that's left a massive impact on their lives and careers.

"It's the end of a long process," he said. "I started my involvement in 2005 as a site assistant and digger on a short-term contract. I was in my late 20s.

"It's 15 years later and I'm in my early 40s and I'm a senior lecturer at Newcastle University. It's been with me a long time – it's part of my career. For all the other people in the report it's been a huge part of our lives."

News of the discovery at Grange Farm also comes as Saxon graves and items from Iron Age, Roman and Medieval times were discovered on a nearby building site in Rainham.

Work was halted recently in the High Street, where Churchill Retirement Living is building a block of flats, after the important archaeological finds came to light.

The site is a few doors down from the Manor Farm pub, where early Iron Age and Roman pottery was discovered when the Premier Inn Hotel was built in 2010.

Churchill Retirement Living was granted planning permission for 54 apartments in 2019 and work began to knock down four bungalows last summer.

A company spokesman said: “The work on our site in Rainham has brought to light some features of archaeological significance, so having liaised with the relevant authorities we have brought in a team of professional archaeologists and temporarily paused construction while they carry out their important work."

Evidence of early Saxon funerary activity has been uncovered at the site. Although no bones remain, teeth were found, and the position they were found in led archaeologists to believe they have hit upon a grave site.

Objects discovered so far include Roman coins, iron knives, flint arrowheads and more than 3,000 sherds of pottery.

The spokesman added: "The archaeologists have so far found features from the early, middle and late Iron Age, and the Roman, Saxon and Medieval periods and it is possible that the site was continuously utilised by people throughout all those periods.

"The most significant discovery is a number of early Iron Age saltern hearths, which would have formed part of the processing of salt in this period.

"We are proud to be enabling these discoveries.”