The Civitas for the Catuvellauni. It is also the only known British Roman town to have been granted Municipium status. It was sacked in the Boudobrican revolt and the inhabitants massacred.
http://en.wikipedia.org/wiki/Verulamium
http://roman-britain.co.uk/places/verulamium.htm
http://www2.rgzm.de/transformation/unitedKingdom/Towns/Pic07.htm