Before meeting Lindsey Davis, I had a vague notion that Oxford played some part in the English Civil War, but wasn’t sure of the facts. I had heard that a statue on the porch of St Mary the Virgin on the High Street is pockmarked because it was shot at by Oliver Cromwell’s troops and I had heard of a mysterious passageway between Christ Church and Merton created during the bitter struggle between Parliamentarians and Royalists, but I was hazy on the details.

Lindsey — whose latest book, Rebels and Traitors, is a sprawling epic set during the bloody 17th-century conflict — is quick to reassure me that I’m not the only one. “I don’t think it’s understood just how important Oxford was in the Civil War,” she says, explaining that Charles I chose Oxford as his headquarters during the four-year struggle.

“He had his parliament in Oxford. The town was absolutely crammed with soldiers and courtiers for those years. The university pretty much closed down. Young ladies of the court would appear wearing the fashions of the day, outraging elderly dons. It must have been a lively time for the city.”

Davis sets her narrative not just in Oxford, but also London and Birmingham. These were crucial settings for the war, but she also has a personal connection to each city: she lives in London, but was raised in Birmingham and studied English at Lady Margaret Hall in Oxford.

She found herself visiting Oxford a good deal over the three years she researched her narrative, visiting the Ashmolean to see the armour-plated hat that King Charles I’s trial judge wore (“one of the most wonderful curiosities of English history”) and the Museum of Oxford, where she spent many a rapt hour peering into its display cabinets.

“Doing this book did remind me of my days as a student in Oxford,” she says, “But with the difference that I was now actually doing some work and not just lazing about.”

She paints a picture of a city that was, like the country itself, torn in two; the university was, broadly speaking, Royalist and the town was Parliamentarian. “Nothing about the Civil War was ever clear-cut,” she says. ‘There were townspeople who supported the king and university people who didn’t.’”

And the passageway? She explains that the King had his court in Christ Church, while the Queen had hers in Merton, but they had a corridor built so that she could secretly come and see him. “She obviously did, because she got pregnant while she was there.”

Other colleges were turned into military camps, became gunpowder factories or were taken over by tailors for the manufacture of army uniforms. Charles made financial demands of all the colleges, laying claim to their silver, which he proceeded to melt, but clearly overstepped the mark when he awarded honorary degrees to his loyal followers. “At that point, there was outrage among the university people. He had to stop doing that.”

Oxford’s role in the Civil War ended when Sir Thomas Fairfax and his Roundhead army laid siege to the city in 1646, although Oxford, by now very much on the losing side, simply handed over its keys. “It was to the Parliamentarians’ credit that they didn’t want to destroy the place. Fairfax, in particular, tried to make sure that the books in the Bodleian Library were protected.”

Her 742-page saga, which centres on one couple who find themselves on opposite sides in the war, is a fresh departure — she is best known for her Falco detective novels, set in ancient Rome.

While she was writing it, her partner of more than 30 years, Richard, was overwhelmed by kidney cancer, and her emotions are still raw. “For the three years that I was writing Rebels and Traitors, those were the three years in which he was very ill. He loved the book. He would be in hospital or convalescing at home and he was demanding more and more of it, which helped me, I think. I knew that if I could persuade him to like it, then that meant it was good. He did die and that was sad, but expected. Almost the last conversation I had with him, I was able to say that my book had been sold to my publisher. It’s all terribly poignant. It still brings tears to your eyes.”

l Rebels and Traitors is published by Century.