City of the Legions
Geoffrey refers to it several times, but means it to be Caerleon-on-Usk, which was called Isca Legionis and Isca Legionum in early times. K.H. Jackson identifies it unhesitatingly as Chester, known by the Romans as Urbs Legionis in Latin. Another possibility of Castleford, which was known as Legiolium in Roman times.
Isca Legionis | The Legend of King Arthur