In a recent short break in Chester, I learnt that it is easy to confuse the Welsh names of Chester and of another town 130 miles away in Wales. It can be hard to decipher medieval texts when it is not clear which of these two important sites is under discussion.
Chester stands at the head of the Dee estuary. The Romans built a large fortress there, naming it Deva after the British name of the river.
Nowadays, Chester is in England, but only a few miles from the border with Wales. As late as the 12th century, one of two alternative Welsh names for Chester was still Deverdoeu. (The Welsh name for the river Dee is Dyfrdwy).
Its other—and more enduring—Welsh name was Caerlleon, literally ‘the fortress-city of the legions’. The colloquial modern Welsh name is the shortened form Caer.
The early English-speaking settlers used a name with the same meaning: Legacæstir. In the 11th century, in a parallel with Welsh usage, the shorter name Chester emerged.
Most of the above information is in A History of the County of Chester: Volume 5 Part 1, the City of Chester: General History and Topography.
Another Roman legionary fort stood on the river Usk 130 miles south of Chester, on the outskirts of modern Newport (Monmouthshire). The Romans named it Isca Augusta after the river Usk.
Around 800, the site was known in Welsh as Cair Legeion guar Uisc (fortress of the legions on the Usk). The modern Welsh name is Caerllion (English: Caerleon).