Archaeologists have discovered new ruins at a historic Roman site in St Albans.

The National Grid was carrying out work to re-lay a gas pipe at the ancient Roman site of Verulamium, which gave archaeologists a chance to look underground.

Where there was previously believed to be a crossroads, archaeologists found evidence for the town house of a wealthy person, which may have featured mosaic flooring. An initial assessment of finds from above the floor suggests the building was demolished in the third century AD.

The team also discovered the corner of a city wall, but with no evidence of a tower as they previously expected. The absence of a corner tower is significant as may suggest that the city walls were built as much for show as for defensive purposes.

Engineering company Amec Foster Wheeler is managing the archaeological recording work on behalf of the National Grid, and the excavation team is from AOC Archaeology group. The work is being monitored by the St Albans Museums team and Historic England to ensure it complies with local policy and national legislation to protect the historic environment.

Simon West, district archaeologist for the council’s museums team, said: “As it turns out our map of the layout of Verulamium is not entirely accurate, so we may have to redraw it.

“We are learning new things all the time about our past.”

Verulamium was the third largest city in Roman Britain, and around half of it stood on what is now Verulamium Park in St Albans. Remains of the walls, defensive ditch, and a Roman villa’s hypocaust (an ancient central heating system) can still be seen.

National Grid is replacing 1.5km of ageing gas mains with new pipes, and dug several deep holes during the project.

Simon said: “Two of the holes have produced significant archaeology which is very exciting.

“One near the park’s running track has hit the very corner of the wall around the Roman city. There does not appear to be any evidence of a corner tower.

“At another hole, close to the museum car park, we have found evidence of the interior of a Roman town house with the remains of what is called an Opus Signinum floor. That is one made up of broken tiles and mortar.”

Verulamium Museum was established following an excavation by Sir Mortimer Wheeler in the 1930s, and further excavations were carried out by Sheppard Frere in the 1950s and early 1960s. Much of the site, which is classified as a scheduled monument, has remained relatively undisturbed for centuries.

Cllr Annie Brewster, the council’s portfolio holder for sport, leisure and heritage, said: “This is another reminder that St Albans is a city with a rich, varied and compelling history.

“To find a previously unknown significant house near Verulamium Museum is a wonderful surprise. The 2.25 miles of city walls, enclosing the third largest Roman city behind Corinium (Cirencester) and Londinium (London), had some of the earliest towers, often built to display power and wealth.

“The lack of a corner tower would suggest some areas of the wall were more for show than defence.”