The town grew around the legionary fortress, which was abandoned in the early 70's. It became the Civitates for the Dumnonii. About 180 the town erected a stone wall. The structure was added to in the mediaeval times, but significant portions of it suvive.
http://roman-britain.co.uk/places/isca_dumnoniorum.htm
http://demolition-exeter.blogspot.com/2010/10/exeters-city-wall-2.html
http://www2.rgzm.de/transformation/unitedKingdom/Towns/Pic48.htm