Work being carried out by Severn Trent as part of its £60m investment in Newark has unearthed new evidence of a defensive ditch that was built to protect the town during the 17th Century.
Engineer Mr Nick Wallace said that when they planned the project they were very much aware of the rich history of Newark that lies beneath the ground and that their work might reveal key information.
"The evidence of the ditch was found in a shaft that is being dug to allow us to replace the sewers and this has also shown the line in which the defensive wall would have run," he said.
Before work started plans were developed with Nottinghamshire County Council and Historic England to deal with any archaeological finds.
He said an archaeologist has been overseeing the works at Queens Road and they had carefully planned the project to have the lowest level of disturbance to anything that might be of historical interest and to make sure that any finds were handled in the most appropriate way.
"We stopped all work to allow the archaeologists in and they were able to tell us that the finds from the ditch include 17th Century salt glazed pottery and a single piece of lead shot either from a pistol or carbine," he said.
"Hopefully the more we uncover the more evidence will be revealed about this crucial period in the town's history."
Severn Trent is currently working in Newark to overhaul the waste and water network for the town.
Mr Wallace said the work was progressing well and they would be continuing to work in the town for the next few years.
"We're ready for any more discoveries as we carry out these vital improvements to the town creating our own history for Newark by leaving a lasting legacy," he said.
Nottinghamshire county council archaeologist Ursilla Spencer said the ditch is cut into heavy clay and would have been very hard to dig.
"This is the first tangible evidence of what Newark and its people went through 370 years ago, right under our feet," she said.
Stuart Jennings, from Warwick University and academic adviser to the National Civil War Centre in Newark, said it was an exciting discovery.
"During the British Civil Wars the town was one of the most heavily defended in England and we have substantial surviving earthworks from the period like the Queen's Sconce fort," he said.
"But we know that much more awaits discovery under the ground. After King Charles lost the north of England at the Battle of Marston the chief threat to Newark would come from this direction so to find such substantial defences and deep ditches to the north of the town is not surprising.
"We also know from documentary evidence that local people were conscripted to do much of the work. The trench was part of a massive network of fortifications which by 1645 kept at bay a vast Parliamentarian and Scots army numbering 16,000. It will be fascinating to see what emerges from a detailed analysis of the objects recovered."