Although no concrete archaeological evidence exists, it's possible that when the 5000 soldiers of the Roman Empire's Second Legion arrived in southwest England c50 AD they discovered that they weren't the first inhabitants of the area now occupied by the city of Exeter.
The Celtic tribe known as the Dumnonii were scattered across Cornwall and Devon where they built a large number of hillforts and settlements, the remains of which are still visible today, crowning the summits of Devon's steep-sided ridges, isolated now and lonely, and ringed with trees bent-double by the strong winds that sweep in from the Atlantic.
Some historians and archaeologists believe that Exeter was founded by the Romans on top of what was once a tribal capital of the Dunomnii, making it one of the oldest cities in Britain. If a settlement did exist on the site of the later Roman fort then it would probably have looked similar to the one shown above left (© RAMM), an artist's impression of an Iron Age settlement that survives as a ring ditch just north of Exeter at Stoke Hill.
It's almost certain that the arrow-like alignment of the current High Street and Sidwell Street unconsciously remembers the ghost of an ancient, Celtic trackway that ran from mid-Devon, over the hills north of Exeter, travelling along the spine of a ridge before dropping down suddenly to the marshy ground surrounding the river below. The promontory upon which the modern city is built has been almost obliterated by 2000 years of development but in prehistory it would've been an ideal location for a large settlement, or oppidum, a Celtic walled enclosure, within easy access of the river, for water and fish, and easily defended from attack.
These ancient British Celts gave Exeter its name. The Romans called their new legionary fortress 'Isca Dumnoniorum', a name derived from the Celtic word 'iske' meaning 'water' and which translates as a whole to 'The Waters of the Dumnonii'. The Celtic name for the city was 'Caer Isc', literally 'stronghold on the water'. The Anglo-Saxon word 'ceaster' probably derives from the Latin 'castra', which in turn meant 'camp'. So by the 9th century Exeter was being called 'Execeaster'