City of the Legions
A city in Britain that was the scene of Arthur’s ninth battle against the Saxons, according to Nennius. As in all of the twelve battles, Arthur was victorious.
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.
See also
Isca Legionis | The Legend of King Arthur