A Legionary fortress with an annex was established in 58 and abandoned in 77. The Civitas Cornoviorum grew up on the site of the fort and the line of the east wall of the fort was used for the town fortifications.
http://roman-britain.co.uk/places/viroconium.htm
http://www2.rgzm.de/transformation/unitedKingdom/Towns/Pic52.htm