The City of the Legion (Urbs Legionis) is, in this context, the Roman legionary fortress at York, the Romano-British Eburacum.
Dere Street began at the fort and ran north to Hadrian’s Wall and beyond. The argument against York is that, according to Welsh sources, the only Roman forts called Cities of the Legion were Chester or Deva and Caerleon or Isca (neither of which could have been the site of a 5th-6th century battle against the English). But to claim the Welsh were ignorant of the fact that York was a legionary fortress seems very doubtful.
A later tower erected over the foundation
stones of Roman York
To begin, we have chieftains such as Peredur son of Efrauc (Efrauc = Eburacum/‘York’) and Peredur son of Eliffer (Eleutherius) Gosgordfawr. Peredur is a Welsh rendering of the Roman rank of Praetor. The governor or legate of Britannia Inferior, that is Northern Britain, was in the later period of praetorian rank.
The Roman emperor Caracalla reviewed the administration of Britain and split the province into two: Britannia Superior in the south, which had a consular governor based at London with two legions, the Twentieth at Chester and the Second at Caerleon. Britannia Inferior in the north had a praetorian governor with only one legion, the Sixth at York, where the governor also resided.
The Romans constructed their first fort at Eboracum in 71 CE. The fort’s rectangular construction consisted of a V-shaped ditch and earthen ramparts with a timber palisade, interval towers and four gateways. It covered about 50 acres of a grid-plan of streets between timber barrack blocks, storehouses and workshops. More important buildings included the huge Principia (Headquarters Building), the Commandant's House, a hospital and baths. The fort was designed to house the entire legion and remained a military headquarters almost to the end of Roman rule in Britain.
The fortifications at York were strengthened around 80 CE by a caretaker garrison while the Ninth Legion campaigned with the governor, Julius Agricola, in Wales and Scotland. The original fort was replaced in 108 CE by a massive stone structure with walls that survived long enough to be incorporated into the defenses of Viking and even later medieval York.
The one thing that makes York somewhat suspect as an Arthurian battle site is the presence there during the Roman period of the camp prefect Artorius, from whose name Arthur derives. It is certainly possible the memory of this Artorius influenced the placement of the Dark Age Arthur at the fort.
On the other hand, a Dark Age Arthur based on the Wall may still have had contacts with or even shared a family relationship with whoever was ruling at York. I have suggested in my blog post on Arthur's genealogy that this may have indeed been the case.