This photo of Exeter Guildhall shows the date AD80, but I’ve never known why. There is some evidence that there was a settlement here as early as 250BC, but the Romans named Exeter Isca around 55 AD.
The visible structure of the guildhall itself dates from the late middle ages and the building has been a guildhall for more than 800 years. It’s possible that even earlier Medieval halls are concealed below ground.