Lurking beneath a rather dull and grey London office block there is a fascinating discovering to be made. After passing through an unassuming entrance at 101 Lower Thames Street and descending a flight of concrete steps, you are greeted by the extensive remains of a Roman bathhouse.

This weekend there has been a rare opportunity to visit the Billingsgate site as part of the Festival of British Archeology.

Although not the biggest bathhouse in Roman Londinium (that was a public one in Huggin Hill in Upper Thames Street) Billingsgate is certainly the best preserved. Likely to have enjoyed private use by the owners of an adjoining Roman villa (which could at some point have been a waterfront inn) you can still see the flues for the hot air used to heat the floor and the various rooms passed through in a very particular order (warm room, hot room and cold room with a bath etc).

The buildings dates from the 2nd to 3rd century AD, but the site was still in use until the early 5th century AD – a time when much of Londinium was in decline. They remained hidden away through the Medieval period and beyond until being discovered in 1848 by workmen building the Coal Exchange on the site.

As the Museum of London points out, the “remains at Billingsgate are important in understanding the fate of late Roman London as it is only one of a few recorded buildings to continue in use into the 5th century” and “it is also a rare survival of a building in situ in the City of London.”

With the recent re-opening of the Roman gallery at the Museum of London, now is a great time to be discovering the traces of Londinium. The displays there are largely unchanged, but have been given added contemporary emphasis as part of a young people’s project called Our Londinium 2012.

Taking the view that people living 2000 years ago have similar priorities to those of today, modern objects and descriptions have been placed next to the existing finds. So look out for contemporary jewellery to show that we still like to wear bling, leather shoes because the material continues to be used today and a picture of the Shard as glass was important then.

Although I think the exhibition does get carried away in parts (do we really need to see a Starbucks paper cup in a museum?), it does illustrate the fact that then as now London was a great trading city. And it was a cosmopolitan one too – just as you hear voices from all over the world today, back in Roman times people would have travelled to Londinium from all over the Empire.