Vespasian (Titus Flavius Caesar Vespasianus Augustus), as depicted in the coin, was the Leader of the II Augusta legion that arrived in Exeter around AD55 and set up their fortified camp on the hill above the river Exe, where a Celtic settlement already existed. The Legion had been a major part of the invasion of Britain under the Emperor Claudius, and had fought across the south of the country, including at Maiden Castle in Dorchester.
Exeter was the westernmost Legionary Roman fortress in Britain, and one of the furthest west outposts of the Empire. The name of the city comes from the river Exe plus the Latin word castra, meaning fort. The Romans, however, knew the city as Isca Dumnoniorum – water or river of the Dumnonii.
There’s an interesting artist’s impression of Roman Exeter on this site.